P0420 on a 2020 Subaru Forester
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 2020 Subaru Forester
Ranked from 593 NHTSA owner complaints filed for this vehicle.
- 1
Engine stress/overheating from TCV (thermo control valve) failure damaging the converter
Medium severityA widely reported TCV defect on this generation Forester causes overheating and repeated stall events; one complaint explicitly reports engine overheating tied to the coolant bypass valve recall, and cumulative heat/misfire stress from this fault cluster can degrade the catalyst.
Reported in 10 NHTSA complaints - 2
Worn or failing catalytic converter
High severityOne complaint has a dealer-confirmed catalytic converter failure on this exact vehicle (Check Engine plus Eyesight faults, later confirmed to be the converter).
Reported in 1 NHTSA complaintParts:catalytic converter - 3
Failing downstream (post-catalyst) O2 sensor
Medium severityA lazy or biased rear sensor can mimic reduced catalyst efficiency even when the converter itself is still serviceable.
Common cause - 4
Exhaust leak ahead of the downstream sensor
Low severityA leak upstream of the rear sensor skews the efficiency comparison the ECM uses to set the code.
Common cause - 5
Long-term misfire from Auto Stop-Start restart faults damaging the catalyst
Medium severityRepeated stall/restart events reported by many owners on this model can send unburned fuel into the exhaust, gradually fouling the catalyst.
Common cause - 6
Oil or coolant contamination of the converter
Low severityCoolant or oil entering the exhaust stream chemically poisons the catalyst and drops its measured 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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