Most Common Subaru Check Engine Codes (Head Gaskets, Oil Consumption, and More)

Why Subaru Check Engine Codes Follow Predictable Patterns

Subaru's horizontally opposed "boxer" engine layout creates some unique maintenance challenges compared to inline or V-configuration engines. The flat layout means the catalytic converters sit very close to the engine, the head gaskets are oriented differently than on other engines, and oil drainage paths work against gravity in certain areas. Subaru uses a relatively small number of engine families across its lineup -- the EJ25, FB20, FB25, and FA20 -- so the same codes tend to appear across the Outback, Forester, Impreza, WRX, and Crosstrek. Understanding which engine is in your Subaru is the first step in diagnosing any check engine code.

P0420 -- Catalyst System Efficiency Below Threshold

P0420 is the most frequently reported code on Subaru vehicles. The boxer engine layout places the catalytic converters very close to the exhaust ports, which subjects them to higher temperatures than on most other vehicles. This proximity causes Subaru catalytic converters to wear out faster, often by 100,000-130,000 miles. On 2006-2012 Outbacks and Foresters with the EJ25 engine, Subaru issued TSBs for premature converter failure. Before replacing the converter ($600-$1,500), check the rear O2 sensor -- a sluggish downstream sensor can mimic converter failure and costs only $50-$80 to replace. On turbocharged models like the WRX, running rich from a boost leak or a failing fuel injector can poison the catalytic converter quickly.

P0171 -- System Too Lean (Bank 1)

P0171 on Subaru vehicles is commonly caused by a vacuum leak, a dirty MAF sensor, or a failing PCV valve. On the EJ25 engine, the intake manifold gaskets are a known leak point, especially on 2000-2009 models. The FB20 and FB25 engines in the 2012+ Impreza, Crosstrek, and Forester use a revised intake manifold design that is less prone to leaks, but the PCV valve integrated into the valve cover can still cause lean conditions when it fails. Check the air intake hose between the MAF sensor and throttle body for cracks -- Subaru's flat engine layout means the intake plumbing takes a longer path and uses more flexible joints that can deteriorate over time. Cleaning the MAF sensor with dedicated MAF cleaner is a quick first step that resolves P0171 roughly 30% of the time on Subarus.

P0301 and P0302 -- Cylinder 1 and Cylinder 2 Misfire

Misfire codes on Subaru boxer engines frequently affect cylinders 1 and 2 (the passenger-side bank). On the EJ25 engine, this is often the first sign of a failing head gasket -- coolant seeping into the combustion chamber causes intermittent misfires that show up as P0301 or P0302, especially when the engine is cold. If misfires are accompanied by white exhaust smoke, coolant loss without visible leaks, or bubbling in the coolant overflow tank, suspect a head gasket. On the FB20/FA20 engines, misfires are more commonly caused by ignition coil or spark plug failure. Subaru recommends replacing spark plugs every 60,000 miles, but the boxer layout makes access difficult -- budget $200-$400 in labor for a spark plug change at a shop. On the WRX and STI, worn ringlands (piston ring lands) from detonation cause misfires on cylinder 4.

P0128 -- Coolant Temperature Below Thermostat Regulating Temperature

P0128 means the engine is not reaching operating temperature quickly enough and is extremely common on Subaru vehicles in cold climates. The thermostat on Subaru boxer engines is a $15-$25 part, but replacing it can cost $200-$400 in labor because of its location on the flat engine. On 2010-2014 Outbacks and Legacy models, Subaru issued a TSB for thermostat replacement with an updated part that opens at a slightly higher temperature. A stuck-open thermostat causes reduced fuel economy, slower cabin heating, and can prevent the engine from entering closed-loop fuel control. On turbocharged Subarus, a stuck-open thermostat also reduces turbo efficiency because the ECU adjusts boost based on coolant temperature.

P0340 -- Camshaft Position Sensor Circuit Malfunction

P0340 on Subaru boxer engines can indicate a failing camshaft position sensor or, more seriously, a timing belt or timing chain issue. On EJ25 engines with timing belts (pre-2012 Outback, Forester, Legacy), a worn timing belt can cause the camshaft timing to drift slightly, triggering P0340 before the belt actually breaks. This makes P0340 an important warning code on EJ25 engines -- if your timing belt has not been replaced at the recommended 105,000-mile interval, do it immediately. The camshaft position sensor itself costs $30-$50 and is located on the back of the cylinder head, making it moderately difficult to access. On the FB20 and FA20 engines with timing chains, P0340 more commonly indicates a sensor failure rather than a chain stretch issue.

P0028 -- Intake Valve Control Solenoid Circuit (Bank 2)

P0028 relates to the AVCS (Active Valve Control System), which is Subaru's variable valve timing system. This code indicates the intake cam on Bank 2 (driver side) is not responding correctly to the oil control valve signal. The most common cause is dirty or sludged engine oil restricting flow to the AVCS solenoid. Performing an oil change with the correct 0W-20 synthetic oil and a new filter resolves this code in many cases. If the code persists, the AVCS oil control valve solenoid ($40-$80) may need replacement. On the FB20 engine, this code can also appear when the engine oil level is low, because the AVCS system relies on oil pressure to operate. Check your oil level first before replacing any parts.

Head Gasket Issues on the EJ25 Engine

The EJ25 2.5L boxer engine used in the 1999-2011 Outback, Forester, Impreza, and Legacy is notorious for head gasket failure. There are two distinct failure modes: external leaks (oil and coolant seeping from the gasket onto the outside of the engine) and internal leaks (coolant entering the combustion chambers). External leaks are more common on the naturally aspirated EJ25 and typically appear between 80,000-150,000 miles. Internal leaks are more common on the turbocharged EJ255/EJ257 in the WRX and STI. A head gasket replacement costs $1,500-$2,500 at an independent Subaru specialist. When replacing head gaskets, always resurface the cylinder heads and use multi-layer steel (MLS) gaskets rather than the original composite gaskets to prevent repeat failures.

FB20/FA20 Oil Consumption

Subaru's newer FB20 (2.0L) and FA20 (2.0L turbo) engines addressed the EJ25's head gasket problems but introduced a new issue: excessive oil consumption. The 2012-2016 Impreza, Crosstrek, and Forester with the FB20 engine can consume one quart of oil every 1,000-3,000 miles. Subaru extended the warranty on some models and offered an oil consumption test -- if the engine fails the test, Subaru will replace the piston rings under warranty. The FA20 in the BRZ/86 is also known for oil consumption, though to a lesser degree. Check your oil level at every other fill-up and keep it within half a quart of full. Low oil in these engines can cause timing chain noise, AVCS codes, and eventually bearing damage.

Tips for Subaru Owners

Subaru vehicles reward owners who follow the maintenance schedule closely. Use the recommended 0W-20 full synthetic oil and change it every 6,000 miles. On EJ25 engines, replace the timing belt and water pump at 105,000 miles without exception -- a broken timing belt will destroy the engine. Independent Subaru specialists typically charge 30-50% less than dealers and often have more experience with common Subaru issues. For parts, Denso, NGK, and Aisin are all OEM Subaru suppliers, and buying their parts directly from auto parts stores saves money compared to dealer pricing. Always check for open recalls and TSBs at subaru.com using your VIN before paying for repairs.

Frequently Asked Questions