P0420 on Subaru
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on Subaru vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 6 Subaru models — most commonly on Crosstrek, Forester, Legacy. P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. Typical repair costs on Subaru range from $900 to $10,000, depending on the specific model and root cause.
Common Causes of P0420
- •Worn-out or failing catalytic converter (Bank 1)
- •Faulty or aged downstream (or upstream) oxygen sensor
- •Exhaust leak before or near the oxygen sensors
- •Engine running rich or lean (fuel trim issues)
- •Misfires or oil/coolant contaminating the converter
- •Damaged or aftermarket low-quality catalytic converter
- •Faulty oxygen sensor wiring or connectors
- •Software/calibration needing update (on some vehicles)
P0420 on Subaru by Model
Subaru Crosstrek(1 issue)
- FB20 Engine Excessive Oil Consumption2013-2017
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.
Subaru Forester(1 issue)
- FB25 Engine Excessive Oil Consumption2011-2018
The 2.5L FB25 engine in 2011-2018 Foresters is widely reported for excessive oil consumption, often burning 1 quart every 1,000-1,500 miles. The root cause is defective piston rings that fail to maintain proper oil control. Subaru acknowledged the issue with TSB 02-157-14R and extended the powertrain warranty to 8 years/100,000 miles for affected vehicles. A class-action settlement (Oakes v. Subaru) covered ring replacement for qualifying owners. The oil consumption test (OCT) required by Subaru involves monitoring oil usage over 3,000 miles at the dealership.
Subaru Legacy(1 issue)
- FB25 Engine Oil Consumption2010-2016
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.
Subaru Outback(1 issue)
- Excessive Oil Consumption2011-2019
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.
Subaru WRX(1 issue)
- Piston Ringland Failure2015-2021
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.
Subaru WRX STI(1 issue)
- Catalytic Converter Efficiency Failure (P0420) on Stock Downpipe2004-2021
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.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on Subaru?▼
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. On Subaru specifically, this code is documented across 6 models.
What causes P0420 on Subaru vehicles?▼
Common causes on Subaru: Worn-out or failing catalytic converter (Bank 1), Faulty or aged downstream (or upstream) oxygen sensor, Exhaust leak before or near the oxygen sensors, Engine running rich or lean (fuel trim issues), Misfires or oil/coolant contaminating the converter. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0420 on a Subaru?▼
Repair costs on Subaru range from $900 to $10,000, depending on the specific model and root cause.
Which Subaru models have P0420 documented?▼
Au7o has documented P0420 on 6 Subaru models: Crosstrek, Forester, Legacy, Outback, WRX, WRX STI.