P0420 on a 2019 Ram 1500 Classic

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 2019 Ram 1500 Classic

Ranked from 1,453 NHTSA owner complaints filed for this vehicle.

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1
Cracked exhaust manifold causing an upstream exhaust leak
High severity
By far the most-reported issue in this batch — repeated reports of cracked 5.7L Hemi exhaust manifolds and broken manifold bolts; the resulting air leak ahead of the catalyst skews the O2 sensor readings the catalyst-monitor relies on.
Reported in 6 NHTSA complaints
Parts:exhaust manifold exhaust manifold bolts
2
EGR cooler failure leaking coolant/exhaust onto the manifold
Medium severity
A failed EGR cooler was reported leaking directly onto the exhaust manifold, which can both introduce an exhaust leak and contaminate/damage the downstream catalyst.
Reported in 1 NHTSA complaint
Parts:EGR cooler
3
Long-term cylinder misfires overheating and melting the catalyst substrate
High severity
A documented cylinder-6 misfire on this vehicle can, left unaddressed, dump raw unburned fuel into the exhaust and cook the converter.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Worn/aged catalytic converter
Medium severity
Catalyst efficiency naturally degrades with mileage; substrate washcoat wears down until it can no longer keep exhaust gases within the monitor's threshold.
Common cause
Parts:catalytic converter
5
Failing downstream (post-cat) O2 sensor
Medium severity
A lazy or biased rear O2 sensor makes the catalyst look inefficient even when the converter itself is fine.
Common cause
Parts:oxygen sensor
6
Oil or coolant contamination of the catalyst
High severity
Oil or coolant reaching the exhaust stream (e.g., from an internal engine leak) coats and poisons the catalyst's active surface.
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