P0420 on a 2021 Ford Escape Hev

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 2021 Ford Escape Hev

Ranked from 326 NHTSA owner complaints filed for this vehicle.

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1
Prior unresolved cylinder misfires / fuel-injector defect damaging the catalyst
High severity
This model has a documented recalled fuel-injector fault with confirmed misfiring; raw unburned fuel reaching the converter from that condition can overheat and degrade the catalyst substrate over time.
Reported in 3 NHTSA complaints
Parts:catalytic converter
2
Coolant loss / overheating contaminating the catalyst
Medium severity
Separate coolant-loss and overheating complaints on this model raise the possibility of coolant reaching the exhaust and fouling the catalyst's coating, lowering its measured efficiency.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Exhaust leak from a missing/failed catalytic converter support bracket
Medium severity
One vehicle had its catalytic converter replaced and the exhaust support bracket left off, causing an exhaust leak that let fumes into the cabin — an exhaust leak ahead of the rear sensor directly undermines the catalyst efficiency reading.
Reported in 1 NHTSA complaint
Parts:catalytic converter exhaust hanger bracket
4
Worn catalytic converter (age/mileage efficiency loss)
Medium severity
The catalyst naturally loses oxygen-storage capacity with age and mileage; this is the code's namesake failure.
Common cause
Parts:catalytic converter
5
Failing downstream O2 sensor
Medium severity
A lazy or degraded rear sensor can mimic reduced catalyst efficiency even when the converter itself is fine.
Common cause
Parts:oxygen sensor

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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