P0420 on a 2019 Honda Civic Hatch

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

Ranked from 414 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from repeated cylinder misfires
High severity
This vehicle has a documented pattern of head-gasket-related misfires (raw fuel/coolant reaching the exhaust), which is a known way to overheat and melt a catalyst's substrate.
Reported in 1 NHTSA complaint
2
Worn or failing catalytic converter
Medium severity
A strong rotten-egg (sulfur) smell, as one owner reports, is a classic sign the converter is no longer processing H2S and has lost efficiency.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Aged/high-mileage catalytic converter (normal wear)
Medium severity
Catalyst efficiency degrades gradually with mileage and heat cycles until it no longer meets the OBD threshold.
Common cause
Parts:catalytic converter
4
Failing downstream O2 sensor
Medium severity
A lazy or biased rear sensor can mimic reduced catalyst efficiency even with a healthy converter; compare upstream/downstream sensor switching rates.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Low severity
An intake air leak in the exhaust path skews the downstream sensor reading toward the upstream signal, tripping the efficiency monitor; inspect gaskets and welds ahead of the sensor.
Common cause
6
Oil or coolant contamination of the catalyst
Medium severity
Consumed oil or coolant reaching the exhaust coats the substrate and poisons the catalyst; check for oil consumption or head-gasket seepage.
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.

Matching symptoms

Check Engine Light