P0420 on a 2016 Dodge Journey

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 Dodge Journey

Ranked from 384 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from chronic misfire (valvetrain / cam-crank sensor wear)
High severity
This vehicle shows a strong pattern of camshaft/crank position sensor failures and rocker-arm/camshaft wear causing repeated stalling and rough running, which is explicitly tied by one owner to catalytic converter efficiency codes appearing alongside cam/crank sensor problems.
Reported in 5 NHTSA complaints
2
Manufacturer catalytic converter defect (Emissions Recall U67)
High severity
One owner's vehicle was subject to an emissions recall specifically covering the catalytic converter on this model.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Catalyst contamination from coolant intrusion (head gasket failure)
Medium severity
A head gasket failure diagnosed on this vehicle would let coolant reach the exhaust stream and coat/damage the catalyst.
Reported in 1 NHTSA complaint
4
Failing downstream oxygen sensor
Low severity
A lazy or degraded rear O2 sensor can mimic reduced catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the sensor
Medium severity
A leak before the rear sensor admits outside air, skewing the efficiency comparison the ECM makes.
Common cause
6
Worn/degraded catalytic converter from normal high-mileage use
Medium severity
Substrate degradation over time reduces oxygen storage capacity below the threshold the ECM expects.
Common cause
Parts:catalytic converter

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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