P0420 on a 2013 Ford Explorer

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 Explorer

Ranked from 2,223 NHTSA owner complaints filed for this vehicle.

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1
Internal (timing-chain-driven) water pump failure mixing coolant into engine oil
High severity
The 3.5L V6 in this generation Explorer uses an internal, chain-driven water pump; when it fails, coolant seeps past the pump seal into the oil pan rather than dripping externally. Owners describe overheating, coolant loss with no visible leak, and diluted/aerated oil — the mixed coolant/oil residue that gets burned or blown by is a documented way to foul a catalytic converter.
Reported in 18 NHTSA complaints
2
Oil visibly diluted/aerated by coolant contamination
High severity
One owner specifically found lighter-colored oil full of air bubbles after a water-pump-related ticking noise — a direct sign of coolant-in-oil contamination that will eventually pass combustion byproducts through the exhaust and degrade the catalyst.
Reported in 1 NHTSA complaint
3
Already-failed catalytic converter reported by owner
High severity
One complaint explicitly states the vehicle failed smog testing due to bad catalytic converters, directly corroborating a P0420 code on this vehicle.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Worn or degraded catalytic converter (age/mileage)
Medium severity
Standard end-of-life washcoat degradation independent of the water-pump issue; confirm with upstream/downstream O2 sensor comparison.
Common cause
Parts:catalytic converter
5
Failing downstream (post-catalyst) O2 sensor
Medium severity
A fouled or lazy rear sensor can mimic reduced converter efficiency, especially plausible given the oil/coolant contamination pattern documented on this vehicle.
Common cause
Parts:downstream oxygen sensor
6
Exhaust leak ahead of the downstream O2 sensor
Medium severity
A leaking manifold gasket or exhaust joint upstream of the rear sensor lets outside air skew the efficiency reading independent of actual converter condition.
Common cause
Parts:exhaust manifold gasket

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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