PO420 problem

Asked by DWDUCK Aug 27, 2018 at 01:06 PM about the 2003 Chevrolet Avalanche 1500 4WD

Question type: Maintenance & Repair

I have 2003 Chevrolet Z71 4wd  5.3L avalanche truck, just
installed new catalyst converters and put new left
upstream oxygen sensor. My engine light turned off but  
still have po420 code. I don’t understand why.

2 Answers
1,115

Jordan answered 6 years ago

Is the P0420 a stored code, or is it a MIL (Malfunction Indicator Lamp) code? You said the light is no longer on, but the computer still shows this code? This leads me to believe it is a stored code for the computer to remember for a couple of KOEO (Key On, Engine Off) cycles in case it sees an issue with the O2 sensors or the catalytic converter. Otherwise, if the light isn't currently on, but the computer has it as a stored code, I wouldn't worry about it at all, and just let the computer do its thing. Hope this helps!

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