P0420 on a 2019 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 2019 Honda Civic Si

Ranked from 356 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Long-term misfires damaging the catalyst (water intrusion at spark plug wells)
Medium severity
This generation Civic has a well-documented design flaw where water enters via the hood scoop/cowl and drips onto the rear spark plug/coil, rusting the coil and plug and causing misfires; owners in this batch had coils and plugs replaced for jerking/shuddering, and sustained misfires are a leading cause of catalyst damage.
Reported in 3 NHTSA complaints
Parts:ignition coil spark plug
2
Failing oxygen sensor
Medium severity
Several owners had the check-engine/emissions light illuminate and were told an emission sensor needed replacement, consistent with a degraded upstream or downstream O2 sensor throwing a false catalyst-efficiency reading.
Reported in 3 NHTSA complaints
Parts:oxygen sensor
3
Head gasket failure allowing coolant into the exhaust stream
High severity
Two owners reported head-gasket failure/coolant flooding the cylinders; coolant reaching the catalyst is a classic way to poison and destroy the substrate.
Reported in 2 NHTSA complaints
Parts:head gasket
4
Rotten-egg (sulfur) smell consistent with catalytic converter failure
Medium severity
A sulfurous smell is the textbook symptom of an overloaded or failing catalytic converter.
Reported in 1 NHTSA complaint
Parts:catalytic converter
5
Contaminated fuel causing a rich condition that stresses the catalyst
Low severity
A dealer specifically diagnosed contaminated fuel and drained the fuel system after a misfire/limp-mode event; running rich on bad fuel accelerates catalyst wear.
Reported in 1 NHTSA complaint
6
Worn/aged catalytic converter (mileage-related efficiency loss)
Medium severity
Catalyst substrates naturally lose efficiency with age and mileage until they no longer clean exhaust well enough to satisfy the OBD-II monitor.
Common cause
Parts:catalytic converter
7
Exhaust leak upstream of the oxygen sensor
Medium severity
A leak ahead of the sensor dilutes the exhaust sample the ECU uses to judge catalyst efficiency, giving a false P0420 reading.
Common cause

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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