P0420 on a 2020 Chevrolet Silverado 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 2020 Chevrolet Silverado 1500
Ranked from 691 NHTSA owner complaints filed for this vehicle.
- 1
Catalytic converter damage from prolonged untreated misfires (AFM/DFM lifter-failure defect)
High severityRaw unburned fuel from a misfiring cylinder (caused by the well-documented lifter/valvetrain failure on these 5.3L/6.2L engines) overheats and can melt the catalyst substrate if driven very far before repair.
Reported in 16 NHTSA complaints - 2
Catalyst contamination from internal engine debris (metal shavings/bearing failure)
Medium severitySeveral owners' engines shed metal shavings or suffered bearing failure internally; debris and oil passing into the exhaust stream is a known way to foul a catalytic converter.
Reported in 2 NHTSA complaints - 3
Worn/degraded catalytic converter (normal aging)
Medium severityCatalysts lose efficiency with mileage regardless of other faults.
Common causeParts:catalytic converter - 4
Failing downstream (post-catalyst) O2 sensor
Medium severityA lazy or degraded rear sensor can mimic reduced catalyst efficiency even with a healthy converter.
Common causeParts:oxygen sensor - 5
Exhaust leak upstream of the downstream O2 sensor
Low severityA leak before the rear sensor skews the efficiency comparison between front and rear readings.
Common cause - 6
Long-term untreated misfires from worn spark plugs/coils melting the substrate
Low severityIndependent of the lifter defect, an uncorrected ignition-side misfire dumps raw fuel into the converter and overheats it.
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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