Tech-Note

The following Toyota factory bulletin refers to codes P0171 and P0174, lean conditions

on either or both banks, but this raises cause for concern when diagnosing catalytic

converter codes as well. This bulletin is acknowledging there may be a problem with

the Intake Air Control Valves on 2004 Camry, Sienna and Solara vehicles with 3.3L

(3MZ-FE) engines.

The same procedure prescribed in this factory publication should be performed before

replacing any catalytic converter on one of the affected vehicles. The reason is that this

potential vacuum leak may be causing higher than acceptable fuel trims, even if not

high enough to set the P0171/P0174 codes which generally appear with fuel trims in

excess of 25%. False catalyst codes will often be set when combined fuel trims on

either bank approach or exceed 10%. This is because the exhaust gas contains too

much oxygen and not enough CO to react with the precious metals on the catalytic

converter substrate. In these cases, the substrate may not get to operating

temperature, or may cool down during the drive cycle. This will allow the exhaust gas

to pass both upstream and downstream O2 sensors "unchanged", triggering a catalyst

code (P0420/P0430). While it is true the catalytic converter was not operating, it would

have likely been the fault of the exhaust mixture entering the converter, not the catalyst

itself. Correct the mixture and the offending catalytic converter code(s) would very

likely disappear as well.

Please see the bulletin below