DTC 0420 Catalyst System Efficiency - Bank 1

[ljbest]

I have a '96 C4 (LT1) that is generating a Diagnostic Trouble Code (DTC) 0420, which indicates that the Left Bank Catalyst is performing below threshold. In looking at other posts on the web, this can result from anything from a faulty gas cap to a bad left-side cat. Performance and mileage are good and the car has 72000 miles. If I clear the code, it comes back on within a day. Does anyone have experience with determining if the cat or o2 sensors (original) are at fault with this code?

Unrelated to this, but a source of angst is a DTC of 1661 which is a fault in the MIL (Malfunction Indicator Lamp) circuit. To mask the DTC 0420, it appears that 'someone' decided to just remove the Service Engine Soon (SES) lamp from the DIC. I suspect just installing a bulb will correct that code reporting. I'd recommend that anyone looking at post-1996 used cars invest in a code scanner and bring it along on test drives.

Thanks,

Larry

 

[WhalePirot]

You're quite a bit above the normal recommended life of your oxygen sensor: about 50Kmiles. Changing it, is where I'd start and it is a rather simple procedure. If you have two sensors, change both.