P0420 on a 2017 Audi Q7
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 Audi Q7
Ranked from 316 NHTSA owner complaints filed for this vehicle.
- 1
Catalytic converter damaged by chronic misfires and oil consumption
High severityThis vehicle has an extensively documented history of oil-burning (piston-ring design defect) and repeated cylinder misfires; sustained misfires combined with oil passing into the exhaust are a well-known way to overheat and poison a catalytic converter's substrate.
Reported in 21 NHTSA complaintsParts:catalytic converter - 2
PCV/crankcase system failure feeding oil into the intake and exhaust
Medium severitySeveral owners had the PCV valve, PCV solenoid, or oil separator diagnosed as failed, a direct pathway for oil to reach and contaminate the catalyst over time.
Reported in 4 NHTSA complaints - 3
Active Grille Shutter fault triggering repeated misfire/limp-mode events
Low severityOne owner's Active Grille Shutter fault (P05A000) repeatedly cascaded into misfires and limp mode per an Audi TSB, adding to cumulative stress on the catalyst.
Reported in 1 NHTSA complaint - 4
Worn or aged catalytic converter
High severityCatalyst efficiency naturally declines with mileage and heat cycling until it drops below the monitored threshold.
Common causeParts:catalytic converter - 5
Failing downstream (post-catalyst) oxygen sensor
Medium severityA degraded rear O2 sensor can mimic reduced catalyst efficiency even with a healthy converter.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the catalyst
Low severityAny leak ahead of the converter skews the front/rear O2 sensor comparison the PCM uses to judge catalyst efficiency.
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.
Rather have a pro handle it? Find a vetted local mechanic.