Q: Q: Cat monitor will not run, even though i've done the 55 mph for 5 minutes or more numerous times....

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I replaced the catalytic converter and a bad o2 sensor about 600 miles ago. All the obd2 readiness monitors are complete except for the catalyst monitor will not run, even though i've done the 55 mph for 5 minutes or more suggested from the GM drive cycle numerous times....?

My car has 235000 miles.
My car has an automatic transmission.

There are two O2 sensors. One before and one after the catalyst. The one before is used for fuel control for fuel injections and the one placed after the catalyst is used to measure catalyst efficiency. A scanner will be able to measure both of them to see if an oxygen sensor is not working correctly. Once you replace an O2 sensor you will need to disconnect the battery in order to reset the KAM and clear it. Fuel control will start from factory setting once this is done as well.

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