P0420 on a 2018 Ford Focus Rs

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 2018 Ford Focus Rs

Ranked from 743 NHTSA owner complaints filed for this vehicle.

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1
Long-term misfires (oil-pump-belt recall defect/valvetrain damage) overheating the catalyst
Medium severity
The same oil-pump-drive-belt recall defect that starved several engines of oil pressure produced misfire symptoms, including one owner's engine 'misfiring significantly'; sustained misfires dump unburned fuel into the exhaust and can overheat and damage the catalyst substrate.
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Coolant/oil contamination of the catalytic converter from head gasket/cylinder head failure
High severity
Reported head gasket and cylinder head failures let coolant reach the combustion chambers and exhaust stream; coolant or oil residue reaching the catalyst is a documented way to poison and degrade converter efficiency.
Reported in 3 NHTSA complaints
Parts:catalytic converter head gasket
3
Worn or failed catalytic converter
High severity
A converter that has aged out or lost internal washcoat efficiency will no longer convert exhaust gases within the monitored threshold.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Low severity
A degraded post-catalyst O2 sensor can report poor catalyst efficiency even when the converter itself is still functional.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leak in the exhaust system before the rear O2 sensor dilutes the sample with outside air, mimicking poor catalyst efficiency.
Common cause
6
Fuel injector fouling causing incomplete combustion
Medium severity
A fouled or leaking injector loads the exhaust with raw or partially-burned fuel, accelerating catalyst wear over time.
Common cause
Parts:fuel injectors

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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