We’ve been told that some Odysseys in these year ranges have exhibited a sticking thermostat. This is a very difficult fault to isolate because it is generally intermittent. The resulting problem is the vehicles will cool off during the drive cycle, and go into open loop.
The ECU is programmed for a richer fuel mixture in open loop, and this will damage a catalytic converter in a short amount of time. We do not know whether Honda has acknowledged this problem yet.
THERMOSTAT OE# 19301-P8F-A11
GASKET OE# 19305-PR7-A00
Some technicians have reported excellent results using an aftermarket thermostat with a slightly warmer heat range of 82C/180F as opposed to the original 78C/172F.
The second problem we’ve been made aware of is the possibility of a malfunctioning Intake Air Temperature Sensor (IAT).
Techs have told us that a telltale sign of a problem with the IAT may be a long term fuel trim reading that does not change from the reading at idle to the reading at 2500 RPM.
The problem again would be an incorrect fuel trim during the drive cycle either causing a too lean or too rich fuel mixture which may or may not set lean/rich codes, but will inevitably cause a catalyst code.
If your long term trim does not change when engine rpms are drastically changed, suspect a problem with the IAT.