P0420 on a 2019 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 2019 Ford Edge

Ranked from 736 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter damaged by a documented history of coolant-intrusion misfires
High severity
This engine has a well-documented coolant-intrusion defect that causes repeated misfires (white smoke, overheating, cylinder coolant contamination); sustained misfire dumps raw fuel into the exhaust and can overheat/damage the converter substrate, which then reads as reduced efficiency.
Reported in 8 NHTSA complaints
Parts:catalytic converter
2
EGR cooler leak contributing to incomplete combustion
Medium severity
The documented internal EGR cooler leak on this engine disrupts combustion quality, which over time can load the converter with unburned fuel/soot and reduce its measured efficiency.
Reported in 1 NHTSA complaint
3
Worn or degraded catalytic converter
Medium severity
Normal aging reduces the converter's oxygen storage capacity below the threshold the PCM checks for during the efficiency monitor.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A drifting or slow-switching rear O2 sensor makes a healthy converter look inefficient on the PCM's comparison test.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Low severity
Outside air entering through a leak ahead of the sensor skews the efficiency comparison even when the converter is fine.
Common cause
6
High-mileage converter nearing end of service life
Low severity
Catalytic converters have a finite service life; on higher-mileage examples natural degradation alone can drop efficiency below the OBD threshold.
Common cause
Parts:catalytic converter

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light