P0420 on a 2015 GMC Terrain
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 2015 GMC Terrain
Ranked from 315 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst damage from prolonged cylinder misfire (timing chain / valvetrain wear)
Medium severityThe same batch shows repeated timing-chain failures and a named dead cylinder; unburned fuel from those misfires can overheat and damage the catalyst substrate over time.
Reported in 5 NHTSA complaintsParts:catalytic converter - 2
Oil consumption contaminating or clogging the catalytic converter
High severityOwners directly describe this engine's oil-consumption defect fouling the catalytic converter and causing stalling, which points to oil-ash poisoning of the catalyst.
Reported in 2 NHTSA complaintsParts:catalytic converter - 3
Carbon/oil sludge buildup restricting exhaust flow
Low severityReported sludge blocking the manifold and head on this engine can also foul downstream exhaust components and sensor readings.
Reported in 1 NHTSA complaint - 4
Worn or aged catalytic converter
Medium severityNormal high-mileage degradation of the catalyst's oxygen storage capacity is the most common P0420 cause industry-wide.
Common causeParts:catalytic converter - 5
Failing downstream oxygen sensor
Medium severityA lazy or biased rear O2 sensor can mimic reduced catalyst efficiency even with a healthy converter; compare front/rear sensor switching.
Common causeParts:oxygen sensor - 6
Exhaust leak ahead of the downstream sensor
Low severityFresh air entering through a manifold or pipe leak upstream of the rear sensor skews the reading toward a false catalyst-efficiency fault.
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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