P0420 on a 2016 Jeep Wrangler
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 Jeep Wrangler
Ranked from 454 NHTSA owner complaints filed for this vehicle.
- 1
Oil or coolant contamination of the catalytic converter from a cracked oil cooler/filter housing
High severityThis engine has a well-documented plastic oil cooler/filter housing that cracks and leaks oil and coolant into the engine bay; fluid drawn into the exhaust stream coats and poisons the catalyst.
Reported in 7 NHTSA complaints - 2
Catalyst substrate damage from chronic cylinder misfire
High severityOwners of this engine report a recurring rocker-arm/lifter valvetrain failure causing persistent misfires; unburned fuel repeatedly dumped into the exhaust overheats and can melt the converter's substrate.
Reported in 6 NHTSA complaints - 3
Worn or aged catalytic converter
Medium severitySimple mileage-related degradation of the catalyst's precious-metal coating reduces conversion efficiency over time.
Common causeParts:catalytic converter - 4
Failing downstream (post-catalyst) oxygen sensor
Medium severityA lazy or biased rear O2 sensor can make a healthy catalyst look inefficient; compare front/rear sensor switching rates.
Common causeParts:oxygen sensor - 5
Exhaust leak upstream of the oxygen sensor
Medium severityA leaking manifold gasket or pipe joint ahead of the sensor introduces outside air that skews the efficiency calculation.
Common cause - 6
Intermittent rich/lean fueling faults
Low severityFuel system faults that swing the mixture rich or lean can prevent the catalyst from showing proper oxygen storage capacity.
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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