P0420 on a 2015 GMC Yukon 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 2015 GMC Yukon Denali

Ranked from 606 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst overload from AFM lifter-failure misfires
High severity
This truck shows a strong pattern of AFM lifter collapse causing rough running/misfire (see this vehicle's P0300 history); prolonged unburned-fuel events from that condition are a documented way to overheat and damage the converter.
Reported in 6 NHTSA complaints
Parts:catalytic converter valve lifters
2
Worn/failed catalytic converter
High severity
An independent mechanic directly diagnosed a faulty catalytic converter needing replacement after the check-engine light and abnormal vibration appeared.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Oil contamination of the catalyst
High severity
An owner reported heavy oil consumption with metal particles in the oil, consistent with internal engine wear sending oil/debris into the exhaust stream and poisoning the catalyst.
Reported in 1 NHTSA complaint
4
Failing downstream O2 sensor
Low severity
A drifting rear sensor can mimic reduced catalyst efficiency even when the converter meets spec.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leak lets outside air reach the rear sensor and skews its readings.
Common cause
6
High-mileage substrate thermal aging
Low severity
Normal cumulative heat cycling gradually reduces catalyst efficiency even absent any single failure event.
Common cause
Parts:catalytic converter

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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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light