P0420 on a 2018 Jeep Grand Cherokee Trackhawk

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 Grand Cherokee Trackhawk

Ranked from 685 NHTSA owner complaints filed for this vehicle.

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1
Coolant intrusion from a blown head gasket contaminating the catalyst
High severity
Multiple owners report coolant burning in a cylinder and liquid draining into the engine from head-gasket failure; coolant reaching the exhaust coats and poisons the catalyst.
Reported in 5 NHTSA complaints
Parts:catalytic converter head gasket
2
Catalyst damage from repeated, unresolved cylinder misfires
High severity
Owners describe recurring misfire complaints (including a stored P0304 cylinder-4 misfire and a spark plug diagnosed as misfiring) — raw unburned fuel from a persistent misfire overheats and degrades the converter.
Reported in 4 NHTSA complaints
Parts:catalytic converter
3
Exhaust leak upstream of the downstream sensor (cracked EGR cooler/exhaust components)
Medium severity
A cracked EGR cooler was reported on this vehicle; a leak in that area ahead of the rear sensor can skew catalyst-efficiency readings.
Reported in 1 NHTSA complaint
Parts:EGR cooler
4
Worn/degraded catalytic converter (age and mileage)
Medium severity
Catalyst efficiency declines with normal thermal aging until it can no longer meet the OBD monitor's threshold.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen sensor
Medium severity
A lazy or drifting rear O2 sensor can mimic a catalyst-efficiency fault even when the converter itself is still good.
Common cause
Parts:oxygen sensor
6
Oil contamination of the catalyst
Low severity
Oil reaching the exhaust stream (from a leaking valve seal or gasket) can coat and poison the catalyst washcoat.
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