P0420 on a 2020 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 2020 Ram 1500

Ranked from 613 NHTSA owner complaints filed for this vehicle.

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1
Exhaust manifold leak or crack
High severity
Two owners independently reported an exhaust manifold leak (broken bolts or a cracked manifold) with ticking/rattling noise and exhaust odor — a leak ahead of the sensors is a direct P0420 trigger.
Reported in 2 NHTSA complaints
Parts:exhaust manifold gasket exhaust manifold
2
Catalyst contaminated by coolant from a head gasket failure
Medium severity
One owner had all four head gaskets crack, sending coolant into the engine and oil; coolant/oil combustion byproducts are a known way to poison a catalytic converter.
Reported in 1 NHTSA complaint
Parts:catalytic converter head gasket
3
Catalyst damage from a sustained cylinder misfire
Medium severity
One owner experienced an actual engine misfire with rough idle and stall, and sustained misfires are a well-known way to overheat and damage a catalyst's substrate.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Worn or aged catalytic converter
High severity
Catalyst efficiency naturally declines with mileage and heat cycling until it falls below the monitored threshold.
Common cause
Parts:catalytic converter
5
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic low catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
6
General exhaust leak or loose joint upstream of the sensors
Low severity
Any leak ahead of the catalyst skews the front/rear O2 comparison the PCM uses to judge 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