P0420 on a 2016 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 2016 Chrysler Town And Country

Ranked from 521 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter damage from untreated cylinder misfires
High severity
Owners report mechanic/dealer-confirmed cylinder misfires (including a #5 injector fault and single-cylinder misfires); unburned fuel from repeated misfires overheats and damages the catalyst, dropping its efficiency below threshold.
Reported in 5 NHTSA complaints
Parts:catalytic converter
2
Engine running rich/lean from injector or vacuum leak (accelerated catalyst wear)
Medium severity
A dealer diagnosis on this vehicle called out both a faulty fuel injector and a vacuum harness — either condition upsets the air/fuel ratio in a way that accelerates catalyst degradation.
Reported in 1 NHTSA complaint
Parts:fuel injector vacuum hose
3
Worn/aged catalytic converter (normal high-mileage degradation)
Medium severity
Catalyst efficiency naturally declines with age/mileage until it can no longer meet the OBD threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A degraded downstream sensor can mimic a switching pattern too close to the upstream sensor, falsely indicating low catalyst efficiency.
Common cause
Parts:oxygen sensor
5
Exhaust leak between the catalyst and the downstream O2 sensor
Medium severity
A leak here draws in outside air and skews the downstream sensor reading.
Common cause
6
Oil or coolant contamination of the catalyst from internal engine wear
Medium severity
Oil or coolant reaching the exhaust stream coats and poisons the catalyst substrate, reducing efficiency.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light