P0420

Diagnostic Chart:

Wiring Diagram:

The catalyst monitor test is performed once per drive cycle if all test conditions are met. The main enable criteria to enable a catalyst test are: a steady TP sensor reading and calculated temperature of catalytic converter (using ECT sensor, IAT sensor, load and run time) indicating system is fully warmed up. Once the enable criteria are met, 50 samples of the sensor voltage amplitudes are collected. The results of the 50 samples are averaged and converted to a millivolt (mV) reading which indicates the efficiency of the converter. The catalyst monitor tests consist of two stages. Stage one voltage ranges from -555 mV to a positive 550 mV When this value goes above 0 mV indicating stage one has failed, the stage two test will start to be used in determining the efficiency. When stage two reaches the fail threshold the MIL (SERVICE ENGINE SOON) lamp will be illuminated and a DTC 0420 will be set. The Scan tool can display the current value in millivolts for stage one as well as indicate if the stage one test has passed or failed. Whenever DTCs are cleared or the battery has been disconnected the results of stage one will be reset to a -555 mV. For more information on the catalyst test refer to General Description located in this manual following the P0420 diagnostic chart.

DTC PARAMETERS
The stage one will fail when the catalyst efficiency becomes less than 75% efficient. After stage one fails, the 50 sample test results are used by stage two. When the results of stage two are greater than the fail threshold mV (catalyst efficiency below about 70%), the MIL (SERVICE ENGINE SOON) telltale lamp will be illuminated and a DTC P0420 will be set.

Probable causes of a DTC P0420 are:
^ Exhaust system leaks between the front and rear sensors or rear sensor leaking.
^ Catalyst deteriorated due to excessive time operating vehicle with a catalyst damaging misfire.
^ Contamination of converter due to incorrect sealant, fuel, or fuel additive.

IMPORTANT: A catalyst monitor test will not be performed if a fault exists in the rear or front oxygen sensor circuits.

DIAGNOSTIC AIDS
DTC P0420, once set, should remain active. However, under certain driving conditions the catalyst monitor test may pass. This condition may occur if driving the vehicle at highway speeds (high air flow) but if the P0420 is set and no exhaust leaks are found the catalytic converter should be replaced. If a misfire (P0300) is detected P0420 test will not run. Be sure to correct a misfire condition before replacing the converter.