P0420 on a 2017 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 2017 Honda Civic

Ranked from 574 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Chronic fuel injector failure causing repeated cylinder misfires that damage the catalyst
High severity
A very large cluster of complaints describes recurring cylinder 1-4 misfires traced to failing fuel injectors on the 1.5L engine (known issue, extended warranty disputes); sustained misfires overheat and melt the catalyst substrate.
Reported in 6 NHTSA complaints
Parts:fuel injectors
2
Engine oil dilution (fuel leaking into the oil) producing a chronically rich mixture that fouls the catalyst
Medium severity
Multiple oil-analysis complaints show 10-14% fuel-in-oil on this 1.5T engine, a known defect; the excess fuel reaching the exhaust side over time contaminates the catalyst.
Reported in 5 NHTSA complaints
3
Head gasket failure / coolant contamination of the catalyst
High severity
One owner's head gasket failed twice (110k and again at 36k miles later); another needed an early coolant flush for overheating — both point to coolant reaching the exhaust side.
Reported in 2 NHTSA complaints
Parts:head gasket
4
Turbocharger failure sending oil into the exhaust stream
Medium severity
A turbo/engine replacement was diagnosed after a motive-power loss, a known route for oil to wash into the exhaust and poison the catalyst.
Reported in 1 NHTSA complaint
Parts:turbocharger
5
Worn/aged catalytic converter
High severity
Normal substrate degradation with mileage remains the baseline cause of efficiency-below-threshold codes.
Common cause
Parts:catalytic converter
6
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or degraded rear O2 sensor can mimic a worn cat on the efficiency monitor.
Common cause
Parts:oxygen sensor
7
Exhaust leak ahead of the downstream O2 sensor
Medium severity
A leaking manifold gasket or exhaust joint dilutes the sample the rear sensor reads.
Common cause
Parts:exhaust gasket

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