Catalyst Monitor — What does it tell you

Learn what the catalyst monitor means and what it’s telling you

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 its job, the ECM should see an almost flat line from the downstream oxygen sensor.

To test the efficiency of the catalytic converter, the software will command a series of rapid fluctuations in air-fuel mixtures, from rich-to-lean and lean-to-rich. It will then watch the results of both the upstream and downstream oxygen sensors. If the upstream sensor sees rapid fluctuations in oxygen, the ECM knows the upstream O2 sensor is working properly. If the catalytic converter is doing its job properly, it will clean up the excess fuel and the computer will expect to see a relatively flat line on the downstream O2 sensor.

However, if the catalytic converter has lost its ability to store oxygen, it can’t burn off the excess fuel. That will cause the downstream sensor signal voltage to vary almost as much as the upstream sensor, indicating that the catalytic converter has lost its 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 will 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 importance of the Catalyst Monitor

The ECM is constantly testing the catalytic converter when the engine reaches certain temperatures and RPMS. The catalyst monitor is responsible for policing the performance of the converter. Once those testing conditions are met, the ECM will initiate the test and record the results. If the converter fails the test two times, the ECM will set a catalytic converter trouble code, and the catalyst monitor will show NOT READY.

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 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.

If you’re experiencing a problem with a catalyst monitor not ready condition, see this article on what steps to take to get it ready to test.

©, 2022 Rick Muscoplat