P0420 on a 2019 Honda Civic Hatch Type R
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 Hatch Type R
Ranked from 420 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst contamination from head gasket coolant leak
High severityMultiple owners describe a low-mileage head gasket design flaw letting coolant reach the cylinders (one explicitly "flooded with coolant"); coolant passing through the exhaust is a direct route to poisoning the catalyst.
Reported in 3 NHTSA complaints - 2
Catalyst damage from chronic misfire (ignition coil water intrusion)
Medium severityOwners report the well-known hood-scoop water-intrusion issue rusting the #4 cylinder ignition coil/plug and causing a jerk/shudder misfire; sustained misfire sends unburned fuel into the converter and degrades it.
Reported in 3 NHTSA complaints - 3
Catalyst fouling from fuel injector leak / oil dilution
Medium severityOwners report gasoline mixing into the engine oil badly enough to smell fuel and lose power -- excess unburned fuel reaching the exhaust degrades catalyst efficiency over time.
Reported in 2 NHTSA complaints - 4
Chronic misfire (1.5T engine) contributing to catalyst damage
Low severityOne report of sudden severe misfire and internal engine failure on a related turbo engine illustrates how unresolved misfire on this platform can carry through to the catalyst.
Reported in 1 NHTSA complaint - 5
Worn or aged catalytic converter
Medium severityCatalyst substrates simply lose efficiency with mileage and heat cycling even absent another fault.
Common causeParts:catalytic converter - 6
Failing downstream O2/air-fuel sensor
Medium severityA lazy or degraded downstream sensor reads too similar to the upstream sensor, tripping a false efficiency code with a healthy converter.
Common causeParts:oxygen sensor - 7
Exhaust leak ahead of the sensor
Medium severityAny leak upstream of the sensor introduces outside air and can throw off the efficiency calculation.
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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