P0420 on a 2021 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 2021 Chevrolet Silverado 1500
Ranked from 772 NHTSA owner complaints filed for this vehicle.
- 1
Recurring cylinder misfires damaging the catalyst substrate
High severityOwners documented repeated backfiring and cylinder-specific misfires (traced to spark and lifter/valvetrain wear) on this truck; unburned fuel from ongoing misfires overheats and erodes the catalyst over time.
Reported in 4 NHTSA complaints - 2
Oil consumption contaminating the catalyst
Medium severityOwners reported measured oil consumption as high as 3.5 quarts per 3,000 miles and excessive oil use tied to an engine replacement; oil burned past worn rings/seals coats and poisons the catalyst.
Reported in 2 NHTSA complaintsParts:catalytic converter - 3
Worn or deteriorated catalytic converter
Medium severityNormal aging reduces the catalyst's oxygen storage capacity below the threshold the rear O2 sensor needs to see, the single most common P0420 trigger.
Common causeParts:catalytic converter - 4
Failing downstream (post-catalyst) oxygen sensor
Medium severityA drifting or slow rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is still good.
Common causeParts:oxygen sensor - 5
Exhaust leak between the engine and the rear oxygen sensor
Medium severityA leak upstream of the sensor dilutes the exhaust sample with outside air, making the PCM misread catalyst efficiency.
Common cause - 6
Engine running rich from a fuel or sensor fault
Low severityA persistently rich fuel mixture floods the catalyst with unburned fuel, driving efficiency below the monitor's threshold.
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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