P0420 on a 2018 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 2018 Jeep Wrangler
Ranked from 1,793 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst contamination from a head gasket or coolant leak
High severityHead gasket failures documented on this engine (including a manufacturer bulletin for a coolant leak/overheating issue) let coolant and combustion byproducts reach the exhaust, fouling the catalyst.
Reported in 8 NHTSA complaints - 2
Catalyst substrate damage from prolonged, untreated misfires
Medium severityOwners had repeated single- and multi-cylinder misfire codes (P0300, P0304, P0305) go unresolved for extended periods; raw unburned fuel from ongoing misfires can overheat and damage the catalyst.
Reported in 3 NHTSA complaintsParts:catalytic converter - 3
Intake camshaft/rocker wear causing incomplete combustion
Medium severityA Jeep/Mopar TSB cited by an owner links intake camshaft/rocker wear on this engine to misfire codes, which can send unburned fuel and combustion byproducts into the exhaust and degrade the catalyst.
Reported in 2 NHTSA complaints - 4
Worn or aged catalytic converter
Medium severityEfficiency degrades naturally with age/mileage; compare upstream and downstream O2 sensor activity to confirm.
Common causeParts:catalytic converter - 5
Failing downstream oxygen sensor
Low severityA lazy or degraded post-cat O2 sensor can mimic a converter efficiency fault.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the downstream oxygen sensor
Low severityA leak ahead of the rear sensor lets in outside air, skewing readings to falsely indicate low efficiency.
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.
Rather have a pro handle it? Find a vetted local mechanic.