P0420 on a 2016 Acura Mdx

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 2016 Acura Mdx

Ranked from 360 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Prior cylinder misfire (documented rod-bearing failure pattern) overheating the catalyst
High severity
This engine family has a well-documented connecting-rod-bearing defect that produces cylinder misfires (owners logged a P0303 and a #4-cylinder misfire) before failing outright; raw unburned fuel from those misfire events can overheat and damage the catalyst substrate. Check misfire history in freeze-frame data before replacing the converter.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Oil consumption/burning contaminating the catalyst
High severity
One owner reported a large puff of blue smoke with sudden power loss, consistent with oil being burned and coating the catalyst substrate, which reduces its efficiency; check for blue exhaust smoke and abnormal oil consumption.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Worn or aged catalytic converter
Medium severity
Normal high-mileage thermal aging reduces oxygen storage capacity; compare front/rear O2 sensor waveforms to confirm reduced conversion efficiency.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Low severity
A lazy or drifting rear sensor can mimic a bad catalyst; compare its switching activity to the front sensor before condemning the converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream sensor
Medium severity
A leak between the converter and rear sensor lets in outside air, falsely indicating poor catalyst efficiency; inspect the mid-pipe and flange gaskets.
Common cause
Parts:exhaust gasket
6
EGR-related exhaust flow fault
Low severity
A leaking or stuck EGR valve can upset exhaust gas composition reaching the sensors; check EGR valve operation and associated gaskets.
Common cause
Parts:EGR valve

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