P0420 on a 2016 Jeep 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 2016 Jeep Cherokee
Ranked from 1,400 NHTSA owner complaints filed for this vehicle.
- 1
Worn or degraded catalytic converter
High severityFCA issued Emissions Recall U90 ("Catalyst Efficiency") for this exact model/engine; one owner's scan directly showed a catalyst-system-efficiency code alongside the recall reference.
Reported in 1 NHTSA complaintParts:catalytic converter - 2
Exhaust manifold leak upstream of the catalyst/O2 sensor
Medium severityOne owner tied chronic coolant loss and this code to the same U90 recall, which also references an exhaust manifold defect that can leak raw exhaust ahead of the sensor.
Reported in 1 NHTSA complaintParts:exhaust manifold gasket - 3
Catalyst contamination from excess oil consumption
High severityThis engine has a separate oil-consumption/emissions recall (W90) for the 2.4L Tigershark; burned oil passing through the exhaust can poison and degrade the catalyst.
Reported in 1 NHTSA complaint - 4
Catalyst damage from head gasket failure allowing oil/coolant into the exhaust
High severityA failed head gasket on this model put oil and coolant into a cylinder, and that contamination passing downstream can foul the catalyst.
Reported in 1 NHTSA complaint - 5
Failing downstream oxygen sensor
Medium severityA lazy or degraded rear O2 sensor mimics a failing catalyst by not showing the expected difference from the front sensor.
Common causeParts:oxygen sensor - 6
Prior/ongoing misfires overheating and damaging the catalyst substrate
High severityUnburned fuel from repeated misfires ignites in the converter, melting the substrate and dropping efficiency below threshold.
Common causeParts:catalytic converter - 7
Exhaust leaks elsewhere ahead of the sensor
Medium severityAny leak before the downstream sensor introduces outside air and skews the efficiency comparison the PCM relies on.
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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