P0420 on a 2017 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 2017 Jeep Wrangler

Ranked from 304 NHTSA owner complaints filed for this vehicle.

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1
Cylinder misfire from a defective cylinder head (low compression on one cylinder)
High severity
A documented case found DTC P0302 with a cylinder reading 80psi versus much higher on the others, traced to a bad cylinder head at just 61k miles; a second owner had a diagnosed cylinder-4 misfire that recurred after head repair, and a third reported ticking with power loss — sustained misfire like this overheats and damages the converter.
Reported in 3 NHTSA complaints
Parts:cylinder head catalytic converter
2
Oil/coolant intrusion into a cylinder from a head-related fault
High severity
One owner's Jeep lost power with the oil-change light on, and the shop found oil and coolant cross-contamination — this fouls the catalyst directly.
Reported in 1 NHTSA complaint
Parts:head gasket
3
Worn or aged catalytic converter
Medium severity
Normal high-mileage degradation of the substrate reduces conversion efficiency below the OBD2 threshold.
Common cause
Parts:catalytic converter
4
Failed downstream oxygen sensor
Medium severity
A slow or biased rear O2 sensor can make the PCM flag the converter as inefficient even when it's fine.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking manifold gasket or cracked pipe joint lets outside air skew the sensor reading used to judge cat efficiency.
Common cause
Parts:exhaust manifold gasket
6
Oil consumption from worn valve seals fouling the converter
Medium severity
Gradual oil burning through worn valve guides/seals coats the catalyst and reduces its efficiency over time.
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.

Matching symptoms

Check Engine Light