hobie

'02 C230 M111 PO420 code for catalyst inefficient. I cleared it once, came back after rest stop on a 100 mile drive. Car has had fuel map at "2" setting since car had oversize crank pullley installed at 20k. Car has 93k now. Plugs are fresh MB NGKs. Troubleshooting procedure requested, thanks.

MBTech21

The ECU is looking at rear O2 voltage to determine cat effectiveness (IE: does it vary like the front O2 or stay steady around 400 mv like it should). Look at the rear voltage , if it bounces like the front- then the cat is not doing its job. If you have some other weird voltage (like none or low- then it is propably the rear O2.

hobie

Thanks. O2 sensors are 10k old. Cel has not returned during a week of around town driving. I'll check mv at next set and let you know.