I keep getting CEL code P0420 "catalyst efficiency below threshold" in my 2000 V6 camaro. I bought it used and triedswitching between 87 and 92 octane gas. The valves run a little quieter on the higher octane but I still get the code. The problem occurs only during stop and go city driving. When I get out of town, I can erase the code, drive it to the beach all weekend, and drive back to town without it coming on. However, once I hit city traffic the code pops up within minutes. It's not a big problem but driving around with the CEL light constantly on bugs me. Any ideas for a fix are appreciated.

 

How many miles on it?

 

Almost 68,000.

 

If you know anybody with HPTuners, they could tune it so the code won't pop up anymore. Your car runs fine otherwise?

 

Yes it runs just fine even with the CEL on. I don't know anyone with an HPTuner, but will google it to see what they do.

 

It's just a laptop program to tune cars. It costs a bit, but I've found it well worth it. You can tune pretty much anything you want.

 

The DTC you are getting means that the downstream O2 sensor is keeping close step with the upstream O2 sensor. The downstream sensor should show little activity because it is looking at exhaust cleaned up by the cat. The problem could be due to lazy upstream O2 sensors, cat going bad, exhaust leak before the downstream sensor, bad downstream O2 sensor. Cat not lighting up at slow engine speed, possibly due to AIR system problem. Also, engine out of tune and running rich or misfiring.