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A Diagnostic Manual for Troubleshooting the OBDII Catalytic Converter




Following is a complete Diagnostic Manual in helping you diagnose and troubleshoot problems with a vehicles emission system and the possible replacement of emission parts including the catalytic converter.



OBDII refers to On Board Diagnostics generation two. This is a system that has the ability to test it's various components and systems and tell the driver when a fault has been detected that will cause their vehicle to put out excessive emissions. This system first became available on passenger cars in the 1995 model year and 1996 for light trucks with a gross vehicle weight of under 8500lbs.



The system has the ability to detect malfunctions in many engine systems that can cause exces­sive emissions. Let's look at the individual system checks or monitors and how they work.




The 02 sensors are monitored or checked for proper reference signals and to make sure that the heater circuit is functioning. 02 sensors that are located before the catalytic converter are tested for high and low voltage thresholds and for switching frequency. The switching frequency test will count the number of times that the signal voltage goes beyond the mid point of 450 millivolts during a specified time frame and compares this figure with the information stored in the vehicle's computer. The system will also count the rich to lean transition and the lean to rich transition. This again is checked against the time stored in the vehicle's computer.


For downstream or post converter oxygen sensors the system checks to see that there is little or no voltage fluctuation at all. It fluctuation is detected the Malfunction Illumination Light (MIL) will come on showing the driver that a problem exists. These oxygen sensors are also used in another OBD II monitoring system; The Catalytic Converter Monitor.



The vehicle's 02 sensors are used to determine catalytic converters efficiency.  All the oxygen sensors located pre or before the catalytic converter show a voltage pattern on an oscilloscope of a wave form,  having many peaks and valleys. These peaks and valleys are the varying voltages of the oxygen sensor signaling rich or lean air/fuel ratio mixture. This type of wave form is proper for the 02 sensors that are located before the catalytic converter.

The voltage is generated by a sensor containing a probe assembly that comes in contact with the exhaust stream. This probe assembly is made up of a compound called zirconium dioxide, which is an electrically conductive material that can generate small voltages when in contact with oxygen. The probe tip is also coated with platinum to aid in this process.

The downstream or post converter oxygen sensors measure the amount of oxygen present in the exhaust after the converter. Generally the wave form of a system with a properly operating catalyst will show little or no voltage fluctuation on an oscilloscope.




The catalytic converter is comprised of an internal substrate onto which a chemical washcoat is placed that extends the geometric surface area of the substrate and also acts as an absorbing or storing material for oxygen in the exhaust. Onto this substrate the precious metals are impreg­nated that will cause the catalytic reactions with the engine pollutants.



When the engine burns a lean air/fuel mixture, high amounts of oxygen from the exhaust stream flow through the substrate of the catalytic converter. The washcoat on the substrate absorbs this excess 02 and stores it for use during rich air/fuel mixtures when it is released to help in the oxidation process.

A proper functioning catalytic converter's downstream or post converter oxygen sensor will show little or no fluctuation in voltage. If catalytic converter degradation occurred affecting the oxygen storage capacity of the washcoat a fluctuating voltage signal will be sent to the vehicle's computer which will in turn illuminate the (MIL) Malfunction Illumination Light notifying the driver of a problem. The vehicle's computer has an algorithm stored in it which allows the computer to determine converter efficiency by the amount of oxygen absorbed.





The fuel monitor system controls fuel trim to the engine. The vehicle's computer allows for fine tuning of the air/fuel ratio. The computer actually stores information on fuel trim and compares this to a pre programmed base fuel calculation. Fuel trim can be divided into two types.  Long Term fuel trim and Short Term fuel trim. The Short Term fuel trim applies to the function of temporary changing or Short Term changing of the fuel delivery. Short term fuel trim can only happen in closed loop operation. Long term fuel trim applies to the Long Term effects of the Short Term fuel trim corrections.  In simple terms the vehicles computer will add or subtract fuel trim from the programmed base calculation in accordance with the signals received from the vehicles oxygen sensors.

The computer tries to maintain the air/fuel ratio at 14.7 : 1. This is the air/fuel ratio referred to as the Stoichiometric Ratio. Fuel trim information can be monitored through the Data Link with a scan tool. The scan tool will monitor fuel trim in terms of percentage from 1 to 100 percent.



The EVAP system monitor tests for leaks in the Evaporative Emissions System. The vehicle's computer checks the EVAP which is the system where hydrocarbon vapors emitted from the gasoline or the fuel system are stored into a canister that is loaded with activated charcoal. The system monitor is generally comprised of two components, a purge vane and a fuel tank pressure sensor. In most applications the monitor system will close the EVAP system off from the atmospheric pressure and opens the purge valve during vehicle cruise down. The fuel pressure sensor in the fuel tank then checks the rate at which a vacuum increases in the EVAP system, this information, is then used by the vehicle's computer to determine purge flow rate or volume. Remember, the function of this system is to store hydrocarbon vapor into the charcoal canister and then purge them into the engine, generally through the intake manifold, via a hose that connects the two points together. The purge valve opens at the proper time to allow this to happen. The EVAP monitor also checks the system for fuel leaks. It does this by closing the purge valve completely, which in turn causes the system to be completely closed. The pressure sensor in the fuel tank then checks the leak down rate. If the rate is greater than the value stored in the vehicle's computer, a leak is determined to exist and the MIL will be illuminated.



The EGR monitor works by performing tests, which open and close the EGR valve and measure the voltage signal sent by the EGR valve's position sensor. The computer compares these signals to the ones that are stored in the computer and uses these figures to determine exhaust gas flow and efficiency. If a problem is detected the MIL will illuminate.



This system checks the function of the secondary air injection system components. The diverter valve and bypass valve are checked for proper function and if the system uses an electronically driven air pump instead of the common engine belt driven type it checks this also. The before converter oxygen sensors are used to check the exhaust to detect the excess oxygen that will be present in the exhaust to determine if there is air flow from the air pump. If a malfunction is indicated the MIL will illuminate.



This system monitors and detects misfire on a cylinder by cylinder basis. The system does this by using the crankshaft position sensor to detect a slowdown in crankshaft speed caused by a misfiring cylinder. Not only can the system detect a misfire, it can also isolate and determine which cylinder misfired. This is done through the vehicle's computer monitoring sensor signals from not only the crankshaft position sensor, but also the camshaft position sensor. If a misfire occurs excess hydrocarbons can be released into the exhaust gas, which can damage the catalytic converter.  When the computer detects a misfire it will illuminate the Malfunction Indicator Light!



The OBDII vehicle computer can be invaluable in diagnosing what is causing a vehicle to have emissions problems and MIL illumination. The system was designed with some SAE (Society of Automotive Engineers) guidelines that make the system economical and easy for all technicians to use. The system uses a data connector or link in which all of the pins or connections are the same no matter who the vehicle manufacturer is. This enables a technician to purchase one OBDII reader or scan tool and be able to use this tool on all vehicles. Another guideline called for easy access to the location of this data connector. The guidelines also called out for a stan­dardized system or fault code identifications. This way all vehicle manufacturers would use the same codes to show MIL illuminating DTCs (diagnostic trouble codes).



First you will need a scan tool or code reader to be able to decode or read the diagnostic trouble codes. When you connect the scan tool to the data link and turn it on it will display or read the stored DTCs. These codes appear like:


The first character in the display code identifies what system the code relates to.

B - is used for body codes.

C - is used for chassis codes.

P - is used for powertrain codes.

The second character denotes what type of code it is.


1 - manufacturer.

2 or 3 - SAE

The third character denotes what system the code refers to. 

1 or 2 - denotes fuel or air.

3 - denotes ignition.

4 - denotes emission control.

5 - denotes speed control.

6 - denotes vehicle computer or output.

7 or 8 - denotes transmission.

9 or 0 - denotes SAE.

The fourth and fifth characters denote the part or condition that caused the problem. The scan tool's manual will identify the meaning of all displayed codes.

This explanation is in a simple or abbreviated form and the scan tool manual, vehicle service manual or competent diagnostic manual should be consulted for complete explanation of the OBDII system. The system may also have capabilities to display enhanced DTC as opposed to just generic codes. So again, it is important to have not only proper tools but also service manuals if you truly want to get to understand OBDII inside and out. The DTCs are even broken down into 4 types.

Type 1 or Type A  - indicates a DTC that is an emission related failure that illuminates the MIL after one trip in which the fault is detected.

Type 2 or Type B - indicates a DTC that is an emission related failure that illuminates the MIL after two trips in which the fault is detected.

Type 3 or Type C - indicates a DTC that is a non emission related failure and illuminates the MIL after one trip in which the fault is detected.

Type 4 or Type D - indicates a DTC that is a non emission related failure, that illuminates the MIL after two consecutive trips in which the fault is detected.


Once the MIL has come on it can be turned off with a scan tool. The MIL should only be turned off after the problem that caused it is corrected. Never disconnect the vehicles battery to try to shut the light off, doing so can cause a host of new problems for the car owner or shop. Disconnecting the battery can cause simple but aggravating problems like a radio that will not work unless you know the security code to re-enable it or a possible major problem like disconnecting and reconnecting the battery that causes a voltage surge which can damage the vehicles computer. The OBDII system also has the ability to illuminate the MIL and store a trouble code that the computer will reset and shut the MIL off if after three consecutive trips the fault does not occur again.



The main difference between the two is the oxygen storage capacity of the catalytic converter. The catalyst is comprised of three distinct parts.

1) SUBSTRATE - The structure onto which the catalytic reactants are deposited. This structure is placed in contact with the exhaust gases. The substrate can be in the form of pellets or a monolithic honeycomb. The substrate can be metallic or ceramic.

2) WASHCOAT - This is a solution made up of alumina and other chemicals that stabilize and promote a good catalytic reaction. This coating of chemicals also extends the geometric surface area of the substrate. This extending of the surface area is important because, as this area is increased it allows for more space or sites in which a reaction can take place. The more areas in which a reaction can take place, the better the catalytic reaction and better the converter efficiency. The catalytic washcoat also stores excess oxygen from lean operating cycles and releases them for use during rich cycles.

3) NOBLE or PRECIOUS METALS LOADING - These metals, commonly Platinum, Palladium and Rhodium are the elements that make the catalytic reaction to reduce pollutants. The definition of the word catalyst means something that causes a reaction without itself being consumed or changed in that reaction. These metals can be used together or by themselves. It should be noted that OEM OBDII catalysts contain on average about twice the precious metal loading of their non OBDII counterparts.

Now that you know what comprises a catalytic converter, you can see what is really a very important difference between OBDII and non OBDII catalytic converters. OBDII catalytic converters must have excellent oxygen storage capacity. Other chemicals and compounds are added into the washcoat of OBDII converters to aid in this oxygen storage capacity. If the oxygen storage capacity is not good, the MIL will illuminate. Please refer back to the Catalyst Monitor section of this publication.




The purpose of these sensors is to give the engine speed to the vehicle's computer so that it can send spark at the appropriate time to the correct cylinder. There are three types of CKP (crankshaft position sensor) used. The Hall Effect type, which works by using a slotted wheel and a magnetic sensor. The wheel as it rotates interrupts the magnetic field and this information is sent to the computer. The second type is the Magnetic Pulse Generator which generates it's own signal through magnetic induction. Third is The Optical Sensor type. This type uses a spinning wheel or rotor, a light beam and a photo transistor. As the wheel rotates and cuts through the light beam, a signal is sent.


These sensors signal camshaft position and speed and send this signal back to the car's computer. These sensors are of the same types that are used for crankshaft position sensors.



This sensor sends information to the ECU (Electronic Control Unit) to maintain proper air/fuel mixture. It is located either in the exhaust manifold pipe or catalytic converter. As an oxygen sensor operates, it checks to see if oxygen is present in the system. If no oxygen is detected in the system, this would indicate a rich mixture and the oxygen sensor will read about900mV (millivolts).

When excessive amounts of oxygen are present, the sensor voltage will read about 100 mV. Rich mixture means high readings, lean mixtures mean low readings. Please note that the exhaust temperature must be at least 600 degrees Fahrenheit for the sensor to operate, and the measurements should only be taken with a multimeter that has a 10 meg ohm impedance rating. Failure to follow this rule may cause you to replace the vehicle's ECU. A properly functioning sensor will oscillate between 900mV and 100mV every few seconds.  You can measure this with your multimeter. If the reading does not oscillate or stays fixed, the sensor is defective. Also, note that silicone, from either antifreeze or RTV(silicone) sealant, can clog an oxygen sensor and make it defective. If the car has a coolant leak into the combustion chamber, the sensor should be replaced. When using a silicone sealer make sure it is oxygen sensor safe. Also note, there are two types of oxygen sensors. The Zirconia (the most common type) works by generating a voltage, the Titania type works by varying its resistance. Remember how many things are monitored by the oxygen sensor in the ORD 11 system, this is the most important part in catalyst monitoring.


Many parts are used in the ignition system. This system delivers the voltage or spark to the appropriate cylinder to fire the engine. Any of the following problems can cause the converter to fail or be damaged; Fouled spark plugs, bad spark plug wires, or carbon tracked distributor cap. Make sure all spark plugs and wires are firing properly. A misfiring plug not only wastes gas, but it makes the mixture rich and in time will ruin the converter.  Be sure to check distributor timing and vacuum advance for proper operation. Some cars also use a system called Distributerless or Coil Pack where the ignition coil is connected right to the spark plug and the coil is fired through the computer.


Problems in the fuel injection system can also cause problems with the converter. Fuel injection systems differ greatly. Basically, a fuel injection system uses small electrically controlled solenoids that deliver or inject fuel into the throttle body or into the intake part of the engine. These small solenoids are controlled by a computer, that obtains information from various sensors located throughout the engine. These sensors help the computer to determine how much fuel to inject. If an injector is leaking internally or dribbling fuel into the engine, it may damage the converter. If the oxygen sensor is defective or the map sensor is bad, this can lead to converter damage. If the injection system uses a cold start injector, (which is a small injector that is used to richen the mixture by injecting fuel into the intake air stream when the engine is cold) and if this injector is leaking or dripping, or if its temperature sensor is not operating properly this too, can damage the converter. Because of the many different injection systems used, consult the proper manual when diagnosing this system.


This sensor measures the volume of intake air. They are tested with a special tester or a computeruter scan tool. If they are defective they can cause problems from rough idles to no start and poor fuel economy.

Most of the previously talked about systems and

devices can be tested with the use of a scan tool. There are many types of this tool, so please consult tool makers information for proper hookup.


These sensors tell the ECU how much air is entering the engine as well as the load on the engine. It also monitors barometric pressure. When this sensor fails it can cause a rich condition which can damage the converter. Because testing of these sensors vary between make and model, consult the manual before testing. Generally there are two types; the voltage type and the frequency type. To check the voltage type you would use a voltmeter and a vacuum pump. To check the
frequency type you would use a tack and a vacuum pump.


This valve vents fuel vapors from the charcoal canister into the engine. The vapors are collected into the charcoal canister from the fuel tank. The canister is located by the tank or under the hood.


*Air pumps - Air pumps operate from the vehicle engine through a belt. This pump supplies the air injection needed at the catalyst and at the air manifolds. The air injection system as a whole, aids in the control of harmful carbon monoxide.

*Air manifolds - Air manifolds direct air into the exhaust manifolds to aid in reduction of Carbon Monoxide.  When they go bad it is usually obvious due to the loud exhaust noise coining from under the hood. Check the air manifolds to make sure they are free of rust through or corrosion when cool.

*High temperature hose - This hose routes air from the air pump and diverter valve to the air manifolds and check valves. If this hose is burned, check the exhaust check valve that is in that hose, it may be defective.

*Check Valves - These valves allow air to pass to the various components like the air manifolds, converter. They should only allow the air to pass in one direction. If air passes freely in both directions, the diaphragm is ruptured and the valve is no good. This failure can lead to damage of the air pump, diverter valve or the high temperature hose if left unchecked. If hoses are melted or components like the air pump or the diverter valve are damaged, check the exhaust check valve.

*Converter air tubes - This tube routes air into the catalyst to help aid in the oxidation process.  Check to make sure it is connected and is not rusted through. The only OBDII vehicles using air injection to the converter are the Ford Motor Company trucks made in 1996 with the 4.9L, 5.0L and 5.8L engines.


These valves help to route the air to the exhaust manifold or catalytic converter to assist in maintaining proper emissions. Operation should be checked and a manual should be consulted regarding proper test procedures.


This system routes small amounts of exhaust gases back to the intake manifold to reduce oxides of nitrogen or NOX. Systems vary by design so you need to consult the proper manual for the vehicle you are working on. Some newer systems also use an EGR valve position sensor. If the exhaust system you are working on has one, it must also be checked. While a bad EGR system will not lead to a converter failure, it might be the reason why the vehicle has a NOX problem.


These devices help the computer to control the engine to maintain a proper idle speed. When this device goes bad, it can cause erratic idle speed, stalling, hesitation or engine run-on. This device can be checked with a 9 volt power source and an ohmmeter. Consult the vehicle service manual for proper wiring diagram and test procedure.


These sensors are sometimes called Manifold Temperature sensors or Air Charge Temperature sensors. They monitor the temperature of the air intake, and if defective, they cause the problem of poor overall performance, heavy black exhaust smoke and bad fuel economy. They are tested with an ohmmeter.


This sensor is located on either the water outlet or the engine block. It monitors engine coolant temperature and the computer uses this information to control idle speed, timing and fuel mixture. If this sensor goes bad, it can cause heavy black exhaust smoke, surging or poor fuel economy. In test this sensor with the engine cold, disconnect sensor wire harness. With an ohmmeter measure the resistance across the sensor. Reconnect the sensor harness and run the engine for 3 to 5 minutes until it reaches normal operating temperature. Disconnect the wire harness and again measure resistance across sensor. If the difference between cold and hot readings is less than 200 ohms the sensor is defective.


ECS - (Engine Control System) PCM - (Powertrain Control Module) This is the unit that controls all the functions of the engine and powertrain. Everything from how much fuel, to when a cylinder fires, max engine speed and timing are controlled by this unit. All of the signals from the various sensors are sent to this part and all commands back to the various systems come from it.

There are many other sensors used in modern vehicles like Detonation Sensors, Power Steering Cut Off Sensor Switches, Throttle Position Sensors, Vehicle Speed Sensors. Sensors used to turn cooling fans on. Make sure you have the proper manual for the system you are working on.

This article has only covered the main parts and systems that make the engine operate. This booklet is intended to give you background and insight on how the OBDII system works and how to repair it. Only the proper repair manual and experience can answer all questions.


Since most states have gone to some form of enhanced emissions testing, (i.e. I/M 240) let's discuss the OBDII system and how it relates to the emission test process.  The I/M 240 test is an enhanced test procedure that was designed to demonstrate "real world" driving conditions. Prior to this test procedure most states used a Static or Unloaded test procedure where the emissions were sampled out of the tail pipe without the vehicle being under any load. The I/M 240 procedure measures vehicle emissions for HC, CO and NOX under a loaded condition. The I/M 240 is run on a Dynamometer and it takes 240 seconds. This procedure can show which Vehicles are not meeting the emissions standards in a real and reliable way.


Since the OBDII system monitors emissions through its use of dedicated monitors to check for deteriorated or non-functioning systems or controls. It is very important that any vehicle that comes into your shop under the following conditions gets it's OBDII system checked thoroughly before the system is repaired and the vehicle is emission tested.

  • Vehicle comes in with MIL illuminated.
  • Vehicle comes in with a catalytic converter that is broken up internally.
  • Vehicle has failed a State required emissions test.

After all repairs have been made and any trouble codes have been erased and the MIL has been turned off, you will need to drive the vehicle before any emission tests should be performed. The OBDII system conducts their tests under certain conditions or enabling criteria. These conditions are comprised of various information, for example:

1.       Elapsed time since vehicle engine start up.

2.       Throttle position.

3.       Engine speed.

4.       Vehicle speed.

5.       Engine temperature.

Most of the various systems tested by the different monitors are only conducted after the engine has reached its normal operating temperature.

The computer runs three types of tests:

1)     PASSIVE TEST - These are the tests that monitor a system or component without affecting its operation.

2)     ACTIVE TEST - Where the computer has the monitor produce a test signal so that its response can be checked against stored information.

3)     INTRUSIVE TEST - Where the computer performs system checks that affect vehicle performance and emissions.

After a passive test is run and it fails, the computer then performs active test procedures on the system. It after this test a failure is found, the computer will run an intrusive test. If after the intrusive test produces a failure the computer may not store a trouble code and/or illuminate the MIL until after the failure appears in two consecutive tests. Because of this, it is suggested that you perform the drive cycle or I/M Readiness Drive Cycle to be sure that all systems have been checked and that all I/M Flags are set. These are benchmarks or messages that the OBDII system uses to show that all emission monitors have been run. It is important that all of these I/M Flags are set before the vehicle goes through an I/M 240 test.


  1. Vehicle should be started with the engine cold.
  2. Let vehicle idle until the engine is at normal operating temperature (the engine coolant temperature sensor should read 180 F or higher). Generally 5 to 10 minutes is sufficient.
  3. Put vehicle in gear and idle for 45 seconds.
  4. Accelerate to 45mph with no more than a 1/4 throttle for 10 seconds.
  5. Decelerate to 35-40 mph and run at steady throttle for 1 minute.
  6. Drive between 20-45 mph for 4 minutes but do not operate under
    wide-open throttle.
  7. Decelerate and idle for about 10 seconds.
  8. Accelerate to 55 mph using no more than 1/2 throttle for 10 seconds.
  9. Drive 40-55 mph using steady throttle for 1 1/2 minutes.

Shut the vehicle off when you return to the shop and if time permits, run the cycle again after the vehicle has cooled off. Doing so will ensure that all Flags have been set and all problems have been repaired and that the vehicle will pass an I/M 240 test.


The OBDII system can perform other tests and store other information that cannot be fully detailed in this information. Among some of the other information the system can store is a "Freeze Frame Picture Record". This is information is stored when the vehicle's MIL is illuminated showing air/fuel ratio, fuel trim, fuel injector base pulse width, loop status, mass airflow rate, engine speed, engine load, vehicle speed, map sensor signal and engine cooling temperature. This information is very helpful in fixing OBDII system faults.  For more information on OBDII systems, consult the vehicle manufacturer's service manual or a service publication such as Mitchell or All Data.















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