P0420 on a 2016 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 2016 Ford Focus Rs

Ranked from 1,143 NHTSA owner complaints filed for this vehicle.

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1
Engine oil intrusion into the exhaust from oil-starvation engine damage
High severity
Several owners report white exhaust smoke, misfiring with white smoke, and metal engine debris tied to the 1.0L EcoBoost wet-belt/oil-pump failures under NHTSA campaign 23V905000; oil burned through the exhaust coats and degrades the catalyst.
Reported in 4 NHTSA complaints
2
Long-term misfire overheating and damaging the catalyst substrate
High severity
Unburned fuel from a sustained misfire ignites in the converter, melting the substrate and killing efficiency; confirmed here by a complaint of active engine misfire alongside white smoke.
Reported in 1 NHTSA complaint
3
Worn or aged catalytic converter
Medium severity
Normal thermal aging reduces the catalyst's oxygen-storage capacity below the OBD threshold; a rear O2/air-fuel sensor comparison to the front sensor confirms reduced efficiency.
Common cause
Parts:catalytic converter
4
Failing or lazy downstream (post-catalyst) O2 sensor
Low severity
A sluggish or biased rear sensor can mimic reduced catalyst efficiency even when the converter itself is fine; compare live rear sensor switching activity to the front sensor.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Low severity
A leak between the catalyst and the rear sensor lets in outside air, making the sensor read as if the catalyst isn't converting properly.
Common cause
Parts:exhaust gasket
6
Rich-running condition from a leaking fuel injector overwhelming the catalyst
Medium severity
Excess unburned fuel reaching the converter consumes its oxygen-storage capacity faster than it can recover, showing up as reduced efficiency even on a structurally sound catalyst.
Common cause
Parts:fuel injector

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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