P0420 on a 2017 Honda Pilot

Most common causes on the 2017 Honda Pilot

Ranked from 903 NHTSA owner complaints filed for this vehicle.

1. 1

   Worn/failed catalytic converter

   High severity

   This vehicle had front and rear catalytic converters diagnosed for replacement, and a separate complaint logged an explicit P0430 catalyst-efficiency code that partially cleared with a catalytic cleaner — direct evidence of converter wear/inefficiency.

   Reported in 2 NHTSA complaints

   Parts:catalytic converter
2. 2

   Catalyst efficiency reduced by ongoing cylinder misfire

   Medium severity

   Complaints in this batch document rod-bearing-related knocking and misfire codes on the same vehicles; prolonged unburned-fuel exposure from misfire accelerates catalyst degradation.

   Reported in 2 NHTSA complaints

   Parts:catalytic converter
3. 3

   Failing downstream (post-catalyst) O2 sensor

   Medium severity

   A lazy or failed downstream O2 sensor can mimic reduced catalyst efficiency even with a healthy converter.

   Common cause

   Parts:oxygen sensor
4. 4

   Exhaust leak ahead of the downstream O2 sensor

   Low severity

   A leak drawing in outside air before the sensor skews the reading and can falsely trigger a catalyst code.

   Common cause
5. 5

   Oil contamination of the catalyst

   Medium severity

   Oil consumption/leakage into the exhaust stream (e.g. from valve seals or a failing engine) can poison the catalyst and drop its efficiency.

   Common cause

   Parts:catalytic converter
6. 6

   Worn/aged catalytic converter (normal end-of-life)

   Medium severity

   Catalysts naturally lose efficiency with age and mileage independent of any other fault; an O2 sensor switching test on a scan tool can confirm.

   Common cause

   Parts:catalytic converter

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