CODE P0420

1999 GMC Yukon



October, 24, 2010 AT 9:07 PM

Engine Mechanical problem
1999 GMC Yukon V8 Two Wheel Drive Automatic 137987 miles

what oxygen sensor is having the problem


P0420 Cat Converter


1 Answer



October, 24, 2010 AT 9:45 PM

Hi barberman, Welcome to 2carpros and TY for the donation.

P0420-Catalyst System Efficiency Below Threshold (Bank 1)

Oxygen Sensor (Bank 1; Sensor 1)On left exhaust pipe, before catalytic converter.
Oxygen Sensor (Bank 1; Sensor 2)On left exhaust pipe, after catalytic converter.

A code P0420 may mean that one or more of the following has happened: Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible Solutions: Some suggested steps for troubleshooting a P0420 error code include: Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter

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