P0420 on a 2018 Ram 3500

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 2018 Ram 3500

Ranked from 780 NHTSA owner complaints filed for this vehicle.

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1
SCR catalyst degradation / DEF dosing system fault
High severity
Multiple owners report SCR catalyst problems and DEF-system faults tied to failed emissions tests, and a service visit that recommended replacing the exhaust/aftertreatment hardware — directly consistent with a catalyst-efficiency-below-threshold code on the Cummins aftertreatment system.
Reported in 2 NHTSA complaints
Parts:SCR catalyst DEF injector
2
Catalyst/NOx monitor requiring extended relearn after battery disconnect
Low severity
An owner reports the NMHc catalyst, NOx aftertreatment, and PM filter sensors can take over 1,000 miles to complete their relearn after the battery is disconnected (e.g., for service), which can leave catalyst-related codes set/pending until the monitor finishes.
Reported in 1 NHTSA complaint
3
Worn or aged catalytic converter
Medium severity
Normal high-mileage thermal aging reduces oxygen-storage capacity below the OBD efficiency threshold.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen/NOx sensor
Low severity
A sluggish or biased rear sensor can mimic reduced catalyst efficiency even when the converter is functioning normally.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Low severity
A leak between the catalyst/SCR unit and the rear sensor lets in outside air, skewing the efficiency reading.
Common cause
Parts:exhaust gasket
6
Long-term misfire damaging the catalyst substrate
High severity
Unburned fuel from a sustained misfire overheats and melts the catalyst, permanently killing conversion 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.

Matching symptoms

Check Engine Light