P0420 on a 2018 Chevrolet Traverse
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 2018 Chevrolet Traverse
Ranked from 452 NHTSA owner complaints filed for this vehicle.
- 1
Catalytic converter cracking from a broken exhaust pipe bracket weld
High severityThis batch is dominated by reports of a failed exhaust bracket/weld (GM TSB PIP5681A) letting the converter vibrate against its mounts until it cracks and loses efficiency.
Reported in 8 NHTSA complaints - 2
Exhaust leak upstream of the downstream O2 sensor
Medium severityCorroded or cracked exhaust pipe sections documented in this batch let outside air into the exhaust stream ahead of the rear sensor, skewing its reading.
Reported in 2 NHTSA complaintsParts:exhaust pipe - 3
Downstream oxygen sensor failure secondary to catalyst degradation
Medium severityOne complaint explicitly links catalytic converter failure to a subsequent oxygen sensor failure.
Reported in 1 NHTSA complaintParts:oxygen sensor - 4
Spark plug fouling/misfire accelerating catalyst wear
Low severityThe same complaint reports burnt spark plugs alongside the catalytic converter failure, consistent with sustained misfire overheating the substrate.
Reported in 1 NHTSA complaintParts:spark plug - 5
Worn/aged catalytic converter
Medium severityNormal high-mileage degradation of the catalyst's oxygen storage capacity drops conversion efficiency below threshold.
Common causeParts:catalytic converter - 6
Oil or coolant contamination of the catalyst
Low severityInternal engine leaks that let oil or coolant reach the exhaust stream can poison and destroy the catalyst coating.
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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