P0420 on a 2021 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 2021 Jeep Grand Cherokee

Ranked from 397 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst damage from chronic cylinder misfire
High severity
This batch documents recurring, TSB-linked engine misfires on this exact vehicle -- including one where coolant/oil intermixing triggered a literal misfire code (odiNumber 11575153), and two related complaints of ongoing misfire and overheating (odiNumbers 11727024, 11728882). Prolonged untreated misfire is a well-known way to overheat and damage the catalyst substrate.
Reported in 3 NHTSA complaints
2
Oil or coolant contamination of the converter
Medium severity
This batch documents an oil cooler failure allowing oil to leak (odiNumber 11582309) and a separate Pentastar seal-related oil-consumption issue (odiNumber 11710178) on this same vehicle/engine family, either of which can coat and poison the catalyst if oil reaches the exhaust.
Reported in 2 NHTSA complaints
3
Worn or failed catalytic converter
High severity
The classic root cause of a catalyst efficiency code as the substrate ages and loses conversion capacity.
Common cause
Parts:catalytic converter
4
Failing downstream O2 sensor
Medium severity
A degraded sensor can report a false inefficiency reading even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
Allows outside air into the exhaust stream, skewing the reading toward 'inefficient.'
Common cause
6
Normal high-mileage catalyst aging
Low severity
Efficiency naturally declines with age and mileage.
Common cause

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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