The following is a TSB from Mazda, but a little explanation is in order. The bulletin does not mention P0420 (P0421) or P0430 (P0431) codes, but it does mention fuel adaptation codes , P0171, P0172, P0173 and P0174.
The reason this is important data when diagnosing a catalytic converter is that a leaking intake gasket is a PRIME means for un-metered air to enter the engine causing a false catalyst code. Be ABSOLUTELY CERTAIN these gaskets are not leaking before replacing the converter(s)**. As always, your combined fuel trims should be very close to zero percent,
never approaching or exceeding 10% combined or individually.
The second problem is as follows. The Bank 1 manifold converter (firewall side) requires the alternator to be removed to gain access. This usually means disconnecting the battery to prevent accidental arcing. The problem arises after the job is complete. When the vehicle is started, it will need to re-learn its idle strategy.
Anybody who has watched this process knows that the engine will not run smoothly at first, misfiring and bucking until the idle strategy is learned. During this learning period, the little misfires have been known to blown out the weak spots in the intake manifold gasket which if bad enough can leave you with a vehicle that no longer runs!
**For these reasons, we strongly suggest replacing the intake gaskets before performing any catalyst replacement, whether or not they are found to be leaking at the time of inspection.
MIL ILLUMINATION WITH DTCS P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0171 P0174 P1170 P1173 STORED IN MEMORY
2000 - 2001 MPV with a VIN of JM3LW28**10214853 or lower
Some vehicles may experience MIL illumination with one or more of the following DTCs stored in the PCM memory.
DTC P0300 - P0306 (engine misfire)
DTC P0171 or P0174 (fuel system too lean)
DTC P1170 or P1173 (02 sensor no inversion)
The sealing of the intake manifold gasket may be deteriorated, causing air leakage.The sealing material of the gasket has been modified to eliminate this concern. Customers having this concern should have their vehicle repaired according to the following repair procedure:
Inspect intake manifold area for air leakage.
If air leak is found, replace intake manifold gaskets with modified parts according to the MPV Workshop Manual (section 01-13) INTAKE-AIR SYSTEM REMOVAL/INSTALLATION
If air leak is not found, troubleshoot DTC(s) according to appropriate section of the workshop manual.