P0420 on a 2015 Dodge Dart

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 Dart

Ranked from 510 NHTSA owner complaints filed for this vehicle.

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1
Catalyst contamination from chronic oil consumption (2.4L Tigershark issue)
High severity
Multiple owners of this engine report heavy oil burning and blow-by; oil passing into the exhaust coats and poisons the catalyst, lowering its efficiency independent of age.
Reported in 5 NHTSA complaints
Parts:catalytic converter
2
Worn/failed catalytic converter (recall-affected converter)
High severity
One owner's scan directly returned P0420 tied to the converter, and another had an independent mechanic confirm a faulty catalytic converter needing replacement; owners also note a manufacturer recall (#U90) covering this converter.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Failing downstream O2 sensor
Low severity
A drifting rear sensor can mimic a degraded catalyst even when the converter itself still meets spec.
Common cause
Parts:oxygen sensor
4
Exhaust leak ahead of the downstream sensor
Medium severity
A leak at the manifold or pipe joints lets in outside air, skewing the rear sensor's readings.
Common cause
5
Long-term misfires overheating the substrate
High severity
Repeated unburned-fuel events (see this vehicle's P0300 misfire history) can thermally damage the catalyst over time.
Common cause
6
Rich-running condition from a leaking fuel injector overloading the catalyst
Medium severity
Excess unburned fuel reaching the converter accelerates efficiency loss independent of age.
Common cause
Parts:fuel injector

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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