P0420 on a 2016 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 2016 Ford Edge
Ranked from 883 NHTSA owner complaints filed for this vehicle.
- 1
Coolant intrusion into cylinders overheating/contaminating the catalyst (2.0L EcoBoost engine block defect)
High severityWidely reported cracked cylinder walls on the 2016 Edge 2.0L EcoBoost let coolant leak into the combustion chambers, causing chronic misfires and coolant-fouled exhaust that overheats and poisons the catalyst, tripping P0420; Ford TSBs are cited in several of these complaints.
Reported in 23 NHTSA complaints - 2
Head gasket or water pump coolant leak contaminating the catalytic converter
High severityExternal/internal coolant leaks (blown head gasket, failed water pump) let coolant reach the exhaust stream, which coats and degrades the catalyst's substrate.
Reported in 2 NHTSA complaints - 3
Recurring cylinder misfires from ignition coil failure overheating the catalyst
Medium severityRepeated coil failures dump unburned fuel into the exhaust, which overheats and can melt the catalyst brick over time.
Reported in 1 NHTSA complaintParts:ignition coil - 4
Worn/aged catalytic converter (normal high-mileage wear)
Medium severityCatalyst efficiency naturally degrades with age and mileage regardless of other faults; a scan-tool comparison of upstream vs. downstream O2/AFR readings confirms conversion efficiency loss.
Common causeParts:catalytic converter - 5
Failing downstream (post-catalyst) oxygen sensor
Medium severityA lazy or biased downstream O2 sensor can report efficiency below threshold even on a healthy catalyst; compare sensor switching rates to the upstream sensor.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the downstream oxygen sensor
Low severityA leak in the manifold, flex pipe, or catalyst inlet joint lets outside air skew the downstream sensor reading, mimicking poor catalyst efficiency.
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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