P0420 on a 2015 Jeep Grand Cherokee

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 Jeep Grand Cherokee

Ranked from 1,344 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Coolant intrusion into the engine contaminating the catalyst
High severity
One owner had coolant intrusion diagnosed requiring a cylinder head replacement; coolant burned through the exhaust path coats and degrades the catalyst.
Reported in 1 NHTSA complaint
Parts:head gasket
2
Engine misfire damaging the catalyst
High severity
A reported cylinder 2 misfire on this vehicle sends unburned fuel into the exhaust, which overheats and can permanently damage the catalyst.
Reported in 1 NHTSA complaint
Parts:spark plug ignition coil
3
Worn or aged catalytic converter
Medium severity
Normal degradation reduces conversion efficiency below the OBD-II threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Low severity
A lazy or biased rear sensor can make a healthy catalyst read as inefficient.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking gasket or joint upstream of the rear sensor skews its reading.
Common cause
Parts:exhaust gasket
6
Chronic rich or lean fuel trim condition
Medium severity
Sustained rich/lean operation overworks and eventually poisons the catalyst.
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