P0420 on a 2015 Chrysler Town And Country

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 2015 Chrysler Town And Country

Ranked from 381 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from prolonged untreated cylinder misfires
High severity
This exact vehicle/engine has a well-documented pattern of camshaft-lobe and rocker-arm wear plus injector/spark-plug-driven cylinder misfires; unburned fuel from repeated misfires overheats and fouls the catalyst if not repaired.
Reported in 9 NHTSA complaints
2
Catalyst contamination from coolant intrusion into a cylinder
High severity
One owner found antifreeze mixed into a piston chamber via a failed oil cooler/oil filter housing seal common on this engine; coolant burned through the engine coats and poisons the catalyst.
Reported in 1 NHTSA complaint
3
Worn or degraded catalytic converter
High severity
Normal aging drops the catalyst's oxygen storage capacity below the threshold the ECM expects.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A degraded post-catalyst sensor can mimic poor catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream sensor
Medium severity
A leak between the converter and the rear sensor introduces outside air that skews the efficiency reading.
Common cause
6
Engine running rich or lean, skewing the efficiency monitor
Medium severity
Upstream fuel-trim problems can make a healthy catalyst appear inefficient to the monitor.
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