P0420 on a 2016 Ford Mustang
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 Mustang
Ranked from 367 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst damage from prolonged misfire / coolant-oil contamination (head-gasket failure)
High severityThe same coolant-intrusion/head-gasket failure documented on this engine burns coolant and oil through the cylinders and out the exhaust; owners describe white smoke, misfire codes, and engine replacement - conditions known to coat and prematurely wear the catalyst.
Reported in 10 NHTSA complaints - 2
Valve cover oil leak dripping onto the exhaust manifold
Medium severityThe plastic valve covers on this generation are documented to warp/melt and leak oil onto the exhaust manifold (Ford TSB); chronic oil-on-exhaust exposure can contaminate the catalyst over time.
Reported in 2 NHTSA complaintsParts:valve cover gasket - 3
Chronic cylinder misfire from a failing ignition coil accelerating cat wear
Medium severityOne owner's recurring cylinder-3 misfire was traced to a faulty ignition coil; unburned fuel from an ongoing misfire is a direct cause of catalyst overheating and damage.
Reported in 1 NHTSA complaint - 4
Worn/aged catalytic converter (normal wear)
Medium severityIndependent of any specific defect, catalyst efficiency naturally declines with age and mileage.
Common causeParts:catalytic converter - 5
Failed downstream O2 sensor
Medium severityA downstream sensor that no longer tracks properly reads as a failing converter even when the cat itself is fine.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the downstream O2 sensor
Medium severityA leaking joint or gasket ahead of the sensor introduces outside air and skews the efficiency comparison.
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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