Rick's Free Auto Repair Advice

Catalyst Monitor — I/M Catalyst Readiness Monitor

Catalyst Monitor – What does it do?

The catalyst monitor checks the efficiency of the catalytic converter by monitoring the upstream (pre-cat) and downstream (post cat) oxygen sensors. The ECM expects to see large fluctuations in the upstream oxygen sensor. But if the catalytic converter is doing it’s job, the ECM should see an almost flat line from the downstream oxygen sensor.

To test the efficiency of the catalytic converter, carmakers will command a series of rapid fluctuations in air fuel mixtures. It will then watch the results of both the upstream and downstream oxygen sensors. If the upstream sensor reports the rapid fluctuations in oxygen, the ECM knows it’s working properly. It then expects to see a relatively flat line on the downstream sensor.

However, if the catalytic converter has lost its ability to store oxygen, the downstream sensor signal voltage will vary almost as much as the upstream sensor, indicating that the catalytic converter has lost it’s ability to clean up the exhaust. In that case, the ECM will set a catalyst efficiency trouble code.

Many people think a P0430 or P0420 trouble code is caused by a faulty oxygen sensor. That’s almost always false. If the upstream sensor doesn’t detect the rapid change in oxygen during the test, the ECM would set an upstream sensor trouble code and cancel the test. Likewise, if the downstream sensor detects the variation in oxygen levels, that, in itself means the sensor is working.

The Catalyst Monitor is a “Two-Trip” Monitor. If a fault is found on the first trip, the computer temporarily saves the fault in its memory as a Pending Code. The computer does not command the MIL on at this time. If the fault is sensed again on the second trip, the computer commands
the MIL “On” and saves the code in its long-term memory.

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Posted on by Rick Muscoplat

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