P0420 on a 2020 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 2020 Acura Rdx

Ranked from 339 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst damage from repeated misfire events during turbo/intercooler moisture-intrusion limp-mode incidents
High severity
The same large cluster of owners reporting recurring misfire-driven limp mode (rain/humidity, CEL flashing, NHTSA recall 23V-858/TSB 20-053) represents repeated unburned-fuel events that can overheat and degrade the catalytic converter over time.
Reported in 13 NHTSA complaints
Parts:catalytic converter
2
Coolant/oil contamination of the catalytic converter from a head gasket leak
High severity
Two owners had dealer-diagnosed head gasket leaks (one at cylinder 4); coolant or oil reaching the exhaust stream is a known way to poison/damage a catalytic converter.
Reported in 2 NHTSA complaints
Parts:catalytic converter head gasket
3
Worn/aged catalytic converter
Medium severity
Normal catalyst efficiency decline with age and mileage until it fails the onboard monitor threshold.
Common cause
4
Failing downstream oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic poor catalyst efficiency; compare upstream vs downstream sensor switching activity.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Low severity
A leak between the converter and rear sensor lets in outside air, skewing the efficiency comparison.
Common cause
6
Long-term untreated misfires from worn ignition components
Medium severity
Chronic misfires from tired spark plugs/coils dump unburned fuel into the converter and accelerate substrate wear.
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