P0420 on a 2015 Honda Cr V

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 Honda Cr V

Ranked from 668 NHTSA owner complaints filed for this vehicle.

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1
Oil contamination of the catalyst from excessive oil consumption
Medium severity
Two owners in this batch needed 2-3 quarts of oil added between service intervals; oil burned through the combustion chamber coats and poisons the catalyst over time.
Reported in 2 NHTSA complaints
2
Catalyst overheating from a chronic cylinder misfire
High severity
One owner's CR-V had a documented cylinder-#3 misfire with air leakage that persisted through a spark-plug/valve-adjustment attempt; sustained misfire dumps unburned fuel into the exhaust and cooks the substrate.
Reported in 1 NHTSA complaint
Parts:ignition coil spark plug fuel injector
3
Worn or aged catalytic converter
Medium severity
Most common real-world cause of P0420 as these vehicles pass 100k miles; converter efficiency degrades with heat-cycle aging.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A degraded or lazy rear O2 sensor reads a fuel-trim swing similar to a dying converter, triggering a false or genuine P0420.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leaking manifold gasket, flex-pipe, or cracked mid-pipe lets outside air reach the rear sensor and skews the efficiency reading.
Common cause
Parts:exhaust gasket
6
Extended rich-running condition from a fuel-trim or injector fault
Medium severity
Excess unburned fuel from a rich condition accelerates catalyst wear and can push efficiency below the monitor threshold.
Common cause
Parts:fuel injector
7
Degraded upstream (pre-catalyst) O2 sensor skewing fuel trim
Low severity
An aging front sensor slows to respond and biases the ECU's fuel control, indirectly reducing catalyst efficiency.
Common cause
Parts:oxygen sensor
8
Aftermarket or damaged exhaust component (non-OEM cat, dented pipe)
Low severity
A non-OEM converter or a dented/collapsed section restricts flow and rarely meets the OEM efficiency threshold.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light