How can I tell when my cat converter is going bad? 98 blazer 4dr 2wd

 

Mine was rattling! There is a pressure difference test that can be done but I've never done that.

 

By monitoring the ability of the converter to absorb and retain oxygen. This can be done by plotting post-converter O2 sensor readings and directly comparing to pre-converter O2 sensor readings at 2000 rpm. If the sensor cycling time matches (or nearly matches), then the converter is shot. Also you can look at the lag time across the converter during dramatic fuel mixture changes. This is how your PCM knows to turn on the SES light!

This thread below discusses it in detail - especially see posts 25, 30, and 43

https://blazerforum.com/forum/2nd-ge...0-99042/page3/

In any case, Fun-V's misfire from plugs/wires apparently had damaged the converter and then continued to cause an occasional intermittent P0420, even after the converter was replaced. So he had more than one problem, the bad converter being the result of the first. I believe he got all fixed and never had any more issues. Maybe he can weigh in if he reads this. I thought it was very interesting stuff we went through together and he did some very good screen captures that can be used as examples of a bad converter. Also I learned that off-scale O2 sensor readings are not really valid for testing of cycle time.