P0420 on a 2016 Ram 1500

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 2016 Ram 1500

Ranked from 1,508 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst wear from repeated stalling/misfire events
Medium severity
Owners report repeated stalls and rough running tied to crankshaft position sensor faults; sustained rough-running events can accelerate catalyst degradation over time.
Reported in 4 NHTSA complaints
2
Exhaust leak ahead of the downstream oxygen sensor (loose/broken manifold bolts)
High severity
One owner describes a well-documented TSB issue with broken exhaust manifold bolts on this engine; the resulting leak lets outside air skew the downstream sensor and trigger a false efficiency code.
Reported in 1 NHTSA complaint
Parts:exhaust manifold gasket
3
Worn or aged catalytic converter
Medium severity
Catalysts lose efficiency with age and mileage even absent another fault, a common baseline cause of this code.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A lazy or drifted rear O2 sensor can mimic reduced catalyst efficiency on a converter that is otherwise fine.
Common cause
Parts:oxygen sensor
5
Oil or coolant contamination of the catalyst
Medium severity
Coolant or oil reaching the exhaust stream (from an unrelated internal leak) coats and poisons the catalyst surface, reducing efficiency.
Common cause
6
Rich-running condition overloading the catalyst
Medium severity
Excess unburned fuel from a fueling fault chemically overwhelms and prematurely wears the catalyst.
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