P0420 on a 2015 Acura Rdx

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 2015 Acura Rdx

Ranked from 407 NHTSA owner complaints filed for this vehicle.

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1
Long-term misfire damaging the catalyst substrate
High severity
Owners report ongoing misfire-type symptoms (bad-gas-like miss on acceleration, limp mode with idle shake); unburned fuel reaching the converter during repeated misfire events overheats and degrades the substrate.
Reported in 2 NHTSA complaints
2
Catalyst contamination from internal oil consumption
High severity
This vehicle had oil found in cylinders 1/3, metal debris in the oil filter, and white smoke from the tailpipe -- oil burned through the exhaust can coat and poison the catalyst washcoat over time.
Reported in 1 NHTSA complaint
3
Worn/aged catalytic converter
High severity
The most common overall cause of a catalyst-efficiency code; converters degrade with mileage and heat-cycling over a decade of service.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A lazy or biased post-cat sensor can mimic reduced catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking joint or gasket upstream of the rear sensor introduces outside air that skews the efficiency reading.
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.

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Matching symptoms

Check Engine Light