P0420 on a 2019 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 2019 Ram 1500

Ranked from 1,449 NHTSA owner complaints filed for this vehicle.

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1
Cracked or leaking exhaust manifold (5.7L HEMI)
Medium severity
A well-documented HEMI pattern where the manifold cracks or the bolts break, leaking exhaust ahead of the downstream O2 sensor and skewing the catalyst-efficiency reading.
Reported in 6 NHTSA complaints
Parts:exhaust manifold exhaust manifold gasket exhaust manifold bolts
2
Valvetrain wear/misfire (lifter/VVT solenoid) damaging the substrate
High severity
A documented cylinder-6 misfire traced to lifter/VVT wear puts unburned fuel through the converter, which can overheat and damage the substrate.
Reported in 1 NHTSA complaint
Parts:lifters VVT solenoid
3
EGR cooler coolant leak contaminating the exhaust path
High severity
A cracked EGR cooler leaking coolant onto the manifold introduces coolant into the exhaust stream, which can foul the catalyst.
Reported in 1 NHTSA complaint
Parts:EGR cooler
4
Worn or high-mileage catalytic converter
Medium severity
General wear reduces oxygen storage capacity; confirm with a downstream O2 switching-rate comparison before condemning the cat.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen sensor
Medium severity
A slow or biased rear sensor can falsely indicate poor catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
6
General exhaust leak (gaskets, pipe joints) ahead of the sensor
Low severity
Any secondary air leak between engine and downstream sensor will skew the efficiency calculation.
Common cause
Parts:exhaust gasket

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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