P0420
P0420DESCRIPTION
The catalyst's efficiency is demonstrated in its ability to oxidize CO and HC emissions. The Engine Control Module (ECM) compares the output signals of the front and rear oxygen sensors to determine whether the output of the rear sensor is beginning to match the output of the front oxygen sensor. As the catalyst wears, the rear oxygen sensor's signal trace begins to match the front oxygen sensor's signal trace. That is because the catalyst becomes saturated with oxygen and cannot use the oxygen to convert HC and CO into H20 and C02 with the same efficiency as when it was new. A completely worn catalyst shows a 100% match between front and rear sensor outputs.
FAILURE CONDITION
The efficiency of the catalytic converter is measured by comparing the activity of the front and rear oxygen sensors. The ECM will set a code and the Malfunction Indicator Lamp (MIL) will turn on if the front and rear oxygen sensor signals match more than 60% of the time in two of four 170 second monitoring periods during two driving cycles. The measurements are taken when the following conditions are met:
1. The ECM is operating in closed loop.
2. The engine speed is between 1800 and 3200 RPM.
3. The catalyst temperature is above 702°F (372°C).
4. The canister purge function is greater than 0.9.
5. The vehicle is not shifting gears.
6. The engine load is between 1.4 milliseconds and 4.5 milliseconds.
This code indicates that the catalytic converter has been found to have a low efficiency according to the readings of the front and rear oxygen sensors.
Test Procedure:
