P0420 on a 2019 Honda Accord Hybrid
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 Accord Hybrid
Ranked from 529 NHTSA owner complaints filed for this vehicle.
- 1
Coolant intrusion into the cylinders (head gasket/engine block defect) contaminating the catalyst
High severityAn overwhelming share of this VIN's complaints describe coolant leaking into cylinders or a blown head gasket on the 1.5T engine; the steam and combustion byproducts from that coolant intrusion foul and can accelerate catalyst degradation.
Reported in 22 NHTSA complaints - 2
Repeated ignition misfires (coils/plugs) overloading the catalyst
High severitySeveral owners cycled through coil and spark plug replacements for recurring cylinder misfires; sustained unburned fuel from repeat misfires overheats and can melt the catalyst substrate.
Reported in 4 NHTSA complaints - 3
Fuel pump/injector fault causing a rich condition that washes the catalyst
Medium severityOne owner reported a failed fuel pump and injectors alongside the head gasket issue; excess unburned fuel reaching the converter degrades it over time.
Reported in 1 NHTSA complaint - 4
Worn or naturally degraded catalytic converter
Medium severityCatalysts lose efficiency with age/mileage independent of any other fault; a converter at or beyond typical service life will trip this code on its own.
Common causeParts:catalytic converter - 5
Failing downstream (post-catalyst) oxygen sensor
Medium severityA lazy or biased rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is healthy.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the downstream O2 sensor
Low severityA leak before the rear sensor draws in outside air and skews its readings to look like poor catalyst 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.
Rather have a pro handle it? Find a vetted local mechanic.