P0420 on a 2010 Chevrolet Silverado 1500 Hybrid

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 2010 Chevrolet Silverado 1500 Hybrid

Ranked from 422 NHTSA owner complaints filed for this vehicle.

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1
Oil consumption contaminating the catalytic converter
High severity
This 5.3L's documented oil-burning problem (fouling a spark plug, and the related AFM lifter failures) puts oil ash through the exhaust, which coats and poisons the catalyst over time.
Reported in 5 NHTSA complaints
Parts:catalytic converter
2
Long-term cylinder misfire degrading the catalyst substrate
Medium severity
A persistent unresolved cylinder-6 misfire and a separate cylinder/throttle-body misfire code both push raw fuel into the exhaust, which can overheat and damage the catalyst.
Reported in 2 NHTSA complaints
3
Catalytic converter failure
High severity
One owner explicitly had the catalytic converter fail and replaced; converter failure on a truck with this many reported oil-consumption and misfire issues is unsurprising.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A broken exhaust manifold bolt reported on this truck lets outside air skew the downstream sensor's efficiency reading.
Reported in 1 NHTSA complaint
5
Failing downstream oxygen sensor
Medium severity
A lazy or biased rear O2 sensor can mimic reduced catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
6
Worn or aged catalytic converter (mileage-related)
Medium severity
Normal degradation of oxygen storage capacity with age/mileage is the most common P0420 cause industry-wide.
Common cause
Parts:catalytic converter

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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