P0420 on a 2018 Honda Civic Si

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 Si

Ranked from 598 NHTSA owner complaints filed for this vehicle.

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1
Cumulative misfires from ignition coil water intrusion damaging the catalyst
High severity
Documented coil-pack seal failures letting water into the plug well (one confirmed with DTC P0304 and visible rust) push unburned fuel into the exhaust over time, fouling and overheating the catalytic converter.
Reported in 2 NHTSA complaints
Parts:catalytic converter ignition coil
2
Rich-running condition from a fuel system fault
Medium severity
One owner's scan showed the fuel system reading too rich on a cylinder, a condition that accelerates catalyst wear if left uncorrected.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Repeated stalling/misfiring from the recalled fuel pump straining the catalyst
Medium severity
The open fuel pump recall (23V-858) tied to lost motive power and stalling on this vehicle can produce misfire events that gradually degrade catalyst efficiency.
Reported in 1 NHTSA complaint
4
Worn or aged catalytic converter
Medium severity
Normal mileage-related degradation of the catalyst's precious-metal coating reduces conversion efficiency; confirm with pre/post-cat O2 sensor comparison.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen sensor
Low severity
A sluggish or biased rear O2 sensor can trigger a false efficiency code even with a healthy converter.
Common cause
Parts:oxygen sensor
6
Exhaust leak upstream of the downstream sensor
Low severity
A leaking joint or gasket ahead of the rear sensor skews the catalyst monitor reading.
Common cause
Parts:exhaust gasket

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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