P0420 on a 2016 Jeep Compass

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 Compass

Ranked from 289 NHTSA owner complaints filed for this vehicle.

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1
Worn or failing catalytic converter
High severity
An independent mechanic pulled DTC P0420 directly for the catalyst system on this vehicle, and other owners report catalytic-converter-related recall/replacement issues.
Reported in 3 NHTSA complaints
Parts:catalytic converter
2
Recurrence after a recent catalytic converter/O2 sensor replacement
Medium severity
One owner had the converter and O2 sensor swapped, clearing the light, only for a new drivability fault to appear immediately after — consistent with an improperly seated connector or a marginal replacement part.
Reported in 1 NHTSA complaint
Parts:catalytic converter oxygen sensor
3
Failing downstream O2 sensor
Medium severity
A lazy or biased rear sensor can mimic reduced catalyst efficiency even with a healthy converter; compare upstream/downstream switching rates.
Common cause
Parts:oxygen sensor
4
Exhaust leak ahead of the downstream sensor
Low severity
Outside air reaching the rear sensor skews its reading toward the upstream signal, tripping the efficiency monitor.
Common cause
5
Repeated cylinder misfires overheating the substrate
Medium severity
Unburned fuel reaching the converter from ongoing misfires (this vehicle has a documented crank/cam sensor stalling issue) can overheat and damage the substrate.
Common cause
6
Oil or coolant contamination of the catalyst
Medium severity
Oil or coolant reaching the exhaust coats the substrate and reduces conversion efficiency; check for consumption or head-gasket seepage.
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