Catalyst Efficiency Monitor Description

CATALYST EFFICIENCY MONITOR DESCRIPTION





This monitor checks for deterioration of the catalytic converter by comparing upstream (front) and downstream (rear) oxygen sensor readings. When the signal from the downstream oxygen sensor is similar to the signal from the upstream oxygen sensor, the PCM judges that the catalytic converter has deteriorated which illuminates the MIL and stores a DTC. Under normal closed loop engine operation with a good catalytic converter, the oxygen storage capacity of the catalyst is high, so the voltage of the rear HO2S will switch slowly. With a deteriorated catalytic converter, the rear HO2S will fluctuate rapidly near the same frequency of the front HO2S. Refer to the Illustration showing an efficient catalyst and a deteriorated catalyst.

The efficient catalyst on the left has a switch rate or several inversions across the 0.45 volt threshold on the front HO2S while the rear HO2S has few inversions across 0.45 volts. The inefficient catalyst on the right has the same number of inversions on both the front and rear HO2S.

^ Counting Switches: Front and rear HO2S switches (switches past 0.45 volts from rich to lean) are counted. After the required number of front switches are counted, the front to rear switch ratio is calculated. A new or efficient catalytic converter will have a switch ratio near 0, while a failed catalytic converter will have a switch ratio of 0.8 to 0.9.

^ OBDII Drive Cycles: A moving average is used to judge catalytic converter condition, so it may take up to six OBD-II drive cycles for the MIL to illuminate for a deteriorated catalytic converter.

^ Conditions: This monitor only runs after the following conditions are met: the engine is at normal operating temperature based on good inputs from the ECT or CHT, ambient air temperature is not extreme (based on IAT), the engine is operating at greater than minimum load (based on MAF), vehicle speed is within a specific window (based on VSS), and the vehicle is driven at part throttle acceleration (based on TPS).





Catalyst Efficiency Monitor DTC's

Catalyst Efficiency Monitor Service Tips

^ Switch Ratio: A properly operating front HO2S will have a high switch rate in normal closed loop. With an efficient catalytic converter, the down-stream HO2S will have a low switch rate. The switch ratio is determined by the PCM dividing the number of down-stream switches by the number of upstream switches over a set period of time. As the catalyst ages and deteriorates, the downstream switch rate increases. When the switch ratio increases passed a threshold value approximately 0.75, DTCs P0420 and P0430 will set and illuminate the MIL.

^ P0420 / P0430: When a catalytic converter is replaced, it is not necessary to replace the down-stream HO2S. When removing and reinstalling H02S, use a 22mm wrench to remove the HO2S, and use a crows foot and torque wrench to install the HO2S to 30 +/- 41b-ft (41 +/ - 5 Nm)