P0420 on Subaru: How to Diagnose & Fix

The FB25 2.5L engine is known for excessive oil consumption, sometimes using 1 quart every 1,000-2,000 miles. Subaru extended warranty coverage and offered piston ring replacement for affected vehicles. Most common on 2011-2014 models.

2.5L Outbacks are prone to premature catalytic converter degradation that sets a P0420 (Catalyst System Efficiency Below Threshold) check-engine light, often well before 100,000 miles. On some 2015-2016 2.5L Legacy/Outback models the front exhaust pipe assembly (which houses the front catalyst) can crack at the flexible bellows — Subaru issued voluntary recall/campaign WUJ-95 to replace the assembly. Separately, cars with the FB-series oil-consumption issue can contaminate and prematurely poison the catalyst with burnt oil, accelerating the P0420 failure. Subaru extended catalytic-converter coverage to 10 years/100,000 miles (bulletin WVI-19) for certain vehicles that can set P0420, on top of the federal 8-year/80,000-mile emissions warranty.

On the Subaru boxer engine, Bank 2 is the driver's side (cylinders 2 and 4). P0430 sets when the driver-side rear (downstream) O2 sensor tracks the front sensor too closely, telling the ECM the catalytic converter is no longer scrubbing exhaust efficiently. On the Outback this most commonly appears on the 3.6R H6 (EZ36), which has a separate catalyst per bank; note that 4-cylinder Outbacks typically set the Bank 1 twin P0420 rather than P0430. A well-known Subaru wrinkle: excessive oil consumption on some cars coked the O2 sensors and contaminated the converter, so a P0430 (or P0420) is frequently traced to a lazy/oil-fouled rear O2 sensor rather than a truly dead cat. After ~120k-150k miles the washcoat genuinely degrades and the converter itself fails.

The 2.0L FB20 engine in first-generation Crosstreks (2013-2017) shares the same oil consumption defect as the FB25 used in other Subaru models. Defective piston rings fail to maintain proper oil control, leading to oil burning at rates of 1 quart every 1,500-2,500 miles. Subaru covered this under warranty extension and the Oakes v. Subaru class-action settlement. The issue is most prevalent in 2013-2015 models.

The FA20DIT engine can suffer from piston ringland failure, especially when tuned or running high boost. The ringland (area between piston rings) can crack, causing compression loss and engine damage. This is often related to detonation from improper tuning or low octane fuel.

The 2010-2016 Legacy 2.5i uses the FB25 engine which is affected by the same oil consumption issue as other FB25-equipped Subarus. Defective piston rings allow excessive oil to bypass into the combustion chamber. The Legacy's higher average speeds mean oil consumption can be even more noticeable during highway driving. Subaru's oil consumption warranty extension and the Oakes v. Subaru settlement cover affected vehicles.

Owners of WRX STI models report P0420 catalyst efficiency codes as the front catalytic converter ages, especially on higher-mileage cars or those exposed to rich operation/misfires that damage the substrate. Symptoms are typically a check engine light and failed emissions readiness rather than immediate drivability problems. NHTSA complaints and owner reports commonly cite repeated P0420 returns even after sensor replacement when the catalyst is degraded.

The engine computer can set code P0420, Catalyst System Efficiency Below Threshold (Bank 1), and turn on the check-engine light. Subaru issued a step-by-step diagnostic procedure for this code that checks other possible sources first, such as other stored codes, white exhaust smoke, exhaust leaks and air/fuel sensor response. Applies to: 2017-2018 Forester with the 2.5L engine and 2019-2023 Forester, as listed in Subaru service information bulletin 09-110-23R (revised November 7, 2023). The bulletin does not list the 2014-2018 2.0L turbo (XT).