P0420 on a 2022 GMC Sierra 1500

P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.

Not your vehicle? See general P0420 info.

Most common causes on the 2022 GMC Sierra 1500

Ranked from 534 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Oil contamination of the catalyst from excessive oil consumption
High severity
One owner reports the vehicle stalling with smoke from the engine compartment and documented excessive oil consumption — burnt oil passing through the exhaust coats and poisons the catalyst.
Reported in 1 NHTSA complaint
2
Catalyst damage from a prior rough-running/misfire condition
High severity
The same L87 bearing-wear defect on this model produced at least one case of the engine running rough with loss of power after a prior engine replacement — sustained rough running can overheat and degrade the converter substrate.
Reported in 1 NHTSA complaint
3
Worn/degraded catalytic converter (age/mileage)
Medium severity
Catalyst efficiency drops with age and thermal cycling until it no longer clears the monitor's threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Low severity
A sluggish or biased downstream sensor reads too close to the upstream signal, making the ECU think the converter isn't working even when it still is.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leak between the converter and the rear sensor dilutes the sample the ECU uses to judge catalyst efficiency.
Common cause
6
Chronic rich or lean fuel trim fouling the converter
Medium severity
A long-running fuel delivery fault overheats or contaminates the catalyst well before normal end-of-life.
Common cause

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Usually no drivability issue
Failed emissions test
Sulfur smell in some cases
Reduced power if converter is clogged

What to check before replacing parts

Check for misfire and fuel trim codes first

Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.

Inspect for exhaust leaks ahead of the converter

Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.

Compare upstream and downstream O2 sensor activity

Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.

Check converter temperature and performance

Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.

Hold off on replacing the converter until root causes are ruled out

Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.

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Matching symptoms

Check Engine Light