P0420 on a 2019 Subaru Ascent

Most common causes on the 2019 Subaru Ascent

Ranked from 433 NHTSA owner complaints filed for this vehicle.

1. 1

   Oil consumption/burning contaminating the catalytic converter

   Medium severity

   Widespread reports of oil-burning smoke at every cold start on this engine indicate oil is reaching the exhaust stream, which coats and degrades catalyst efficiency over time.

   Reported in 6 NHTSA complaints

   Parts:catalytic converter
2. 2

   Exhaust leak ahead of the catalyst/sensors from a cracked Y-pipe or manifold weld

   Medium severity

   Cracked welds in the Y-pipe/manifold (a recurring complaint on this model) let outside air into the exhaust stream ahead of the converter, skewing the efficiency reading.

   Reported in 2 NHTSA complaints

   Parts:exhaust manifoldcatalytic converter
3. 3

   Worn/aged catalytic converter (normal wear-out)

   Medium severity

   Catalysts lose efficiency with age/mileage regardless of other faults.

   Common cause

   Parts:catalytic converter
4. 4

   Failing downstream oxygen sensor

   Medium severity

   A degraded rear O2 sensor can falsely indicate reduced catalyst efficiency.

   Common cause

   Parts:oxygen sensor
5. 5

   Catalyst substrate damage from long-term misfires

   High severity

   Repeated unburned-fuel loads from a misfire can overheat and melt the substrate.

   Common cause
6. 6

   Secondary exhaust leaks elsewhere in the system (gaskets, clamps)

   Low severity

   Any leak point that admits outside air past the sensors affects the efficiency calculation.

   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.