P0420 on a 2019 Honda Accord

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

Ranked from 650 NHTSA owner complaints filed for this vehicle.

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1
Coolant contamination of the catalytic converter from chronic 1.5L turbo head-gasket failures
High severity
An unusually large share of this batch describes the well-documented 1.5T head-gasket defect (coolant leaking into the cylinders, white exhaust smoke, cylinder-3 misfires); burned coolant passing through the exhaust poisons the catalyst, and the paired misfires overheat the substrate.
Reported in 18 NHTSA complaints
Parts:catalytic converter head gasket
2
Catalyst-damaging misfires from fuel injectors degraded after the low-pressure fuel pump recall repair
High severity
Several owners report the recall fuel pump replacement was followed by fuel injector failure, misfires, and limp mode -- repeated unburned fuel reaching the converter accelerates substrate damage.
Reported in 4 NHTSA complaints
Parts:fuel injector
3
Worn/failing catalytic converter (age/mileage)
Medium severity
Aged substrate loses oxygen-storage capacity independent of any other failure.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A lazy or biased rear sensor can mimic poor catalyst efficiency.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the rear O2 sensor
Medium severity
A leaking manifold, turbo downpipe joint, or gasket lets outside air reach the rear sensor and skews the reading.
Common cause
6
Turbo boost-related rich/lean fuel trim accelerating catalyst wear
Low severity
Boost or MAF-related air/fuel miscalculation on the turbocharged engine can push fuel trims out of range and stress the cat over time.
Common cause
Parts:mass airflow sensor

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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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Matching symptoms

Check Engine Light