P0420 on a 2013 Ford Escape
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 2013 Ford Escape
Ranked from 2,743 NHTSA owner complaints filed for this vehicle.
- 1
Coolant/oil intrusion into cylinders (1.6L EcoBoost cracked head/block) fouling the catalyst
High severityThis model year's 1.6L EcoBoost has a well-documented cracked cylinder head/block defect that lets coolant leak into the combustion chamber and burn off; the residue coats and poisons the catalyst washcoat, driving efficiency below the P0420 threshold.
Reported in 14 NHTSA complaints - 2
Long-term untreated cylinder misfires melting/damaging the substrate
High severityComplaints describe misfire codes accompanying the coolant intrusion; unburned fuel dumped into a misfiring cylinder ignites in the exhaust and overheats the substrate, permanently killing converter efficiency.
Reported in 2 NHTSA complaintsParts:catalytic converter - 3
Worn or degraded catalytic converter (age/mileage)
Medium severityNormal end-of-life washcoat degradation on a 10+ year old vehicle; confirmed by comparing upstream vs. downstream O2/AFR sensor switching rates.
Common causeParts:catalytic converter - 4
Failing downstream (post-catalyst) O2 sensor
Medium severityA lazy or contaminated rear O2 sensor mimics a bad converter by failing to show the expected reduced switching activity relative to the upstream sensor.
Common causeParts:downstream oxygen sensor - 5
Exhaust leak ahead of the downstream O2 sensor
Medium severityA leaking manifold gasket, flex pipe, or downpipe joint upstream of the rear sensor lets outside air skew the reading, making the PCM see poor efficiency that isn't really there.
Common cause - 6
Upstream O2 sensor drift causing skewed fuel trims
Low severityA slow or biased front sensor can push the engine slightly rich or lean, which itself can look like reduced catalyst efficiency on the PCM's monitor.
Common causeParts:upstream 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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