P0420 on a 2019 Honda Civic

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 Honda Civic

Ranked from 348 NHTSA owner complaints filed for this vehicle.

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1
Water intrusion (hood-scoop design flaw) rusting ignition coils/spark plugs and causing chronic misfires
High severity
Multiple owners report water dripping from the hood scoop onto a rear cylinder, rusting the coil/spark plug and setting a check-engine light; sustained misfires from this condition overheat and damage the catalyst.
Reported in 3 NHTSA complaints
Parts:ignition coils spark plugs
2
Head gasket failure / coolant contamination of the catalyst
High severity
One owner had signs of head gasket failure at 67k miles; another had cylinders flooded with coolant shortly after a dealer repair — both let coolant/combustion gas reach the exhaust side.
Reported in 2 NHTSA complaints
Parts:head gasket
3
Engine oil dilution (fuel in oil) producing a rich condition that fouls the catalyst
Medium severity
Two complaints describe gasoline mixing with the engine oil severely enough to cause a power loss, consistent with the 1.5T oil-dilution issue that can richen combustion and load the catalyst.
Reported in 2 NHTSA complaints
4
Failing downstream (post-catalyst) oxygen/emission sensor
Medium severity
A dealer diagnosed a failed emission sensor after an emission-warning complaint on this model/year.
Reported in 1 NHTSA complaint
Parts:oxygen sensor
5
Worn/aged catalytic converter
High severity
Normal substrate degradation with mileage remains the baseline cause of efficiency-below-threshold codes.
Common cause
Parts:catalytic converter
6
Exhaust leak ahead of the downstream O2 sensor
Medium severity
A leaking manifold gasket or exhaust joint dilutes the sample the rear sensor reads.
Common cause
Parts:exhaust 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.

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

Matching symptoms

Check Engine Light