P0420 on a 2018 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 2018 Honda Civic

Ranked from 598 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Repeated engine misfires overheating/damaging the catalyst
High severity
This VIN population has well-documented recurring misfires (coil-pack water intrusion, injector/oil-dilution failures) that dump raw fuel into the exhaust; sustained misfire is a textbook way to overheat and damage a catalyst, and several of these same complaints report engine failure requiring major repair.
Reported in 3 NHTSA complaints
2
Fuel delivery problems causing a rich mixture that stresses the catalyst
Medium severity
One owner's scan showed a rich reading on cylinder #1, and another's emissions light traced back to a malfunctioning fuel pump -- an over-rich mixture accelerates catalyst wear and can trip the efficiency monitor.
Reported in 2 NHTSA complaints
3
Worn/deteriorated catalytic converter
Medium severity
Normal aging reduces the catalyst's oxygen storage capacity below the PCM's efficiency threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A sluggish or biased downstream sensor makes the PCM think the catalyst isn't scrubbing efficiently even when it is.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream O2 sensor
Low severity
A leaking joint upstream of the sensor dilutes the exhaust sample with outside air and skews the catalyst monitor.
Common cause
6
Oil contamination of the catalyst
Medium severity
Oil burned through worn valve seals or (on this engine) fuel-diluted oil getting past rings coats and poisons the catalyst media.
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