P0420 on a 2017 GMC Acadia Denali

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 2017 GMC Acadia Denali

Ranked from 639 NHTSA owner complaints filed for this vehicle.

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1
Long-term misfire/rough-running history from timing chain and camshaft wear overloading the converter
High severity
This vehicle has documented camshaft/timing-chain wear (codes P0010/P0016/P0018/P0507, and a separate camshaft/timing-chain replacement) that produced sustained rough running; prolonged misfire dumps unburned fuel into the exhaust and can overheat/damage the converter.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Failing oxygen sensor(s) and suspect catalytic converter
Medium severity
One owner reported having to replace O2 sensors and suspecting a catalytic converter issue on this vehicle - a documented pattern of sensor and converter trouble on this platform.
Reported in 1 NHTSA complaint
Parts:oxygen sensor catalytic converter
3
Catalytic converter fouling from excessive oil consumption
Medium severity
This engine has a reported pattern of burning oil quickly enough to require monthly top-offs; oil pulled into the exhaust stream coats and fouls the converter substrate, reducing its measured efficiency.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Worn or degraded catalytic converter
Medium severity
Normal aging reduces the converter's oxygen storage capacity below the efficiency threshold the PCM monitors for.
Common cause
Parts:catalytic converter
5
Exhaust leak upstream of the downstream O2 sensor
Low severity
A leak ahead of the rear sensor lets in outside air that skews the efficiency comparison even with a healthy converter.
Common cause
6
Failing downstream oxygen sensor
Medium severity
A slow-switching or biased rear O2 sensor can make a healthy converter appear inefficient on the PCM's comparison test.
Common cause
Parts:oxygen sensor

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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