P0420 on a 2015 Dodge Durango

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 Dodge Durango

Ranked from 502 NHTSA owner complaints filed for this vehicle.

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1
Catalyst contamination from oil consumption due to oil-filter-housing/valve-train failures
High severity
Cracked oil filter housings causing major oil leaks and subsequent lifter/valve-train damage on this engine can let oil reach and foul the catalyst, cutting efficiency below threshold.
Reported in 2 NHTSA complaints
Parts:catalytic converter oil filter housing
2
Catalyst damage from chronic cylinder misfire (Pentastar valve-train wear)
High severity
Confirmed misfire codes traced to worn rocker arms/camshaft lobes on this engine push raw fuel into the exhaust, which can overheat and damage the catalyst over time.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Worn/aged catalytic converter
Medium severity
Normal mileage-related degradation reduces catalyst efficiency below the OBD threshold.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A degraded post-cat O2 sensor can mimic reduced catalyst efficiency even when the converter is still good.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak lets outside air reach the rear sensor, skewing the efficiency comparison the PCM relies on.
Common cause
6
Engine running rich from a fuel system fault
Low severity
Excess fuel overwhelms the catalyst's ability to oxidize it, lowering measured 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.

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Matching symptoms

Check Engine Light