[old_master]

DTC P0420 is "Catalyst efficiency below threshold bank one". When the ECM runs this DTC, there are 24 conditions that must be met before the DTC will complete its test. If it detects a problem, the DTC will be set and the SES light will come on. Due to the complexity of this DTC, it may or may not mean that the catalytic converter is performing properly. On some ECM's, the parameters for setting the DTC are too narrow, which means the DTC might be set when the converter is actually within specifications. GM has a Technical Service Bulletin that involves widening the parameters on certain vehicles, check with the dealer to see if yours qualifies.

As a vehicle is driven, oxygen sensor reaction time slows down, the more miles, the slower it gets. One of the things the DTC checks for is the reaction time between pre and post catalyst oxygen sensors when fuel mixture is changed by the ECM during the test. P0420 can be set if the oxygen sensors reaction time is slower than it should be. The sensor output voltage might be within specifications, but the ECM has no way of knowing if the sensor reacts at the proper speed, so it has to assume reaction speed is ok. As long as the sensor output voltage is within specs, no DTC will be set for the oxygen sensor. The only way to measure reaction time is with an enhanced scan tool that can "dial in" to the oxygen sensors and display the wave pattern of it.

Clear the DTC's and reset the OBDII, (must be done with a scan tool, not a code reader). Drive the vehicle, if the SES light comes on again, have a qualified technician diagnose the system, don't just throw parts at it, unless of course, money is no object.

If the vehicle has 100,000 miles or more, and the oxygen sensors have not been replaced, they're due for replacement. Replacing them MIGHT satisfy the ECM when it runs the test.