P0420 on a 2010 Ford Edge
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 2010 Ford Edge
Ranked from 672 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst contamination from coolant intrusion (failed internal water pump, 3.5L Duratec)
Medium severityOwners of this vehicle report coolant mixing into the oil/cylinders from the known internal water-pump failure on this engine; coolant burned through the combustion chambers can coat and poison the catalyst.
Reported in 2 NHTSA complaints - 2
Catalyst damage from prolonged, uncorrected ignition-coil misfire
Medium severityOwners on this vehicle report coil failures reaching the point of burning up and needing plug/coil replacement; unburned fuel from a sustained misfire overheats and can melt the catalyst substrate.
Reported in 2 NHTSA complaints - 3
Worn or degraded catalytic converter (age/mileage)
Medium severityThe most common root cause of this code as the catalyst's oxygen-storage capacity simply degrades over time and miles.
Common causeParts:catalytic converter - 4
Failing downstream (rear) oxygen sensor
Low severityA lazy or drifting rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is still good.
Common causeParts:oxygen sensor - 5
Exhaust leak ahead of the downstream oxygen sensor
Low severityA leak that lets outside air reach the rear sensor skews its reading toward looking like poor catalyst efficiency.
Common cause - 6
Fuel or ignition system fault causing an over-rich/lean condition that overworks the converter
Low severityAny unmetered fuel trim error forces the catalyst to work harder than it can sustain, accelerating efficiency loss even on an otherwise healthy converter.
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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