P0171 on Subaru: How to Diagnose & Fix

How to Fix

Replace head gaskets with updated MLS (multi-layer steel) gaskets. Ensure the machine shop resurfaces cylinder heads to proper flatness. Replace timing belt, water pump, and thermostat while engine is apart. Use Subaru coolant conditioner to help prevent future failures. Budget $1,500-2,500 for a quality repair.

Owner tips & cautions

• TipDIY repairs can save significantly - dealer charges $1,500-2,500 but DIY costs are typically 50-70% less
• TipUse Subaru coolant conditioner to help prevent future failures.
• WarningThis is a high-severity issue - ignoring it can lead to costly repairs or safety concerns. Address it promptly.
• TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes

How to Fix

1) Check the oil level correctly first. Park on level ground and wait at least 5 minutes after shutting the engine off. Use the correct oil grade; Subaru's bulletin lists 0W-20. Under the bulletin, a dipstick reading at or below the add mark 4,000 miles or more after the last oil change, or a low-oil light 5,000 miles or more after it, does not require an oil consumption test. 2) If the level drops sooner than that, ask a Subaru dealer to run a Vehicle Coverage Inquiry on your VIN for bulletin 02-157-14R. On the Outback, it applies to 2013-2014 manual-transmission cars and to 2013 CVT cars below VIN D*321435. 3) The dealer runs an oil consumption test. The car qualifies if it burns more than 1/3 quart (10.6 oz) in 1,200 miles. 4) The fix for a qualifying engine is a replacement short block, not a new set of rings. For the 2013-2014 Legacy/Outback the bulletin lists part 10103AC600, now superseded by 10103AC650. 5) Coverage: the extension pays for oil-consumption repairs for 8 years/100,000 miles from the original warranty start date. For 2013-2014 cars that period has now ended for nearly every vehicle, so a repair today will probably be paid out of pocket unless the dealer's coverage check shows otherwise. The settlement's cash-reimbursement claim deadline was October 1, 2016. 6) For Outbacks the bulletin does not cover, or that pass the test, look at the conditions the bulletin says raise normal oil use first: the wrong oil grade or viscosity, hard driving, heavy loads or towing, long idling, and extreme temperatures.

Owner tips & cautions

• TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
• TipSearch Subaru Outback forums and owner groups for real-world experiences and DIY guides
• TipGet multiple quotes from independent mechanics - dealer prices can be 2-3x higher for the same repair

How to Fix

Find the exact source of the leak before approving any repair. Subaru bulletin 02-192-24R (Engine Oil Leak Diagnostic Procedures, all FA and FB engines, revised April 2025) has the technician compare the VIN with a table of production fixes. Vehicles older than the listed VIN breaks get every area inspected. A white spray coating (Spotcheck SKD-S2) can help show where fresh oil is coming from. The areas to inspect are: - where the timing chain cover meets the cylinder head, block, cam carrier and cam caps - the cam cap and cam carrier seams at the rear of the heads - the joint between the oil pan and the upper oil pan - the joint between the upper oil pan and the block, near the transmission - the oil level switch area. If oil is found on the oil level sensor, the sensor must be replaced. A leaking joint is cleaned and resealed with new liquid gasket. Subaru says to use its special tools J-51972 and J53038 when removing and refitting the timing chain cover, so the cover is not damaged and seals properly. The bulletin also shows where liquid gasket goes at the ends of the head gasket. The related bulletin 02-131-12R says that if no oil droplets are found, the technician should clean the area and check again at the next service. Replace the head gaskets only once the leak is confirmed to come from the head-to-block seam itself. Ask the shop to show you where the leak is. Use proper Subaru coolant. This problem is less common than on the older EJ engines, but it still occurs. Coverage: neither bulletin is a recall or a warranty extension. Bulletin 02-192-24R sends warranty claims to Subaru's Labor Time Guide. Bulletin 02-131-12R covers 2012-2019 Forester and Impreza and 2015-2019 WRX. It allows the oil-seepage repair to be claimed only while the vehicle is within the Basic New Car Limited or Powertrain warranty period.

Owner tips & cautions

• TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
• TipSearch Subaru Outback forums and owner groups for real-world experiences and DIY guides
• TipGet multiple quotes from independent mechanics - dealer prices can be 2-3x higher for the same repair

How to Fix

Replace both head gaskets with updated MLS (multi-layer steel) gaskets (Subaru OEM 11044AA770 or Six Star MLS set). Resurface both cylinder heads to within 0.002 inch flatness spec. Replace timing belt, water pump (21111AA370), thermostat, and all seals while the engine is apart. Add Subaru Cooling System Conditioner (SOA635071) after repair. Budget 8-12 hours labor for the complete job.

How to Fix

Replace both head gaskets with updated MLS (multi-layer steel) gaskets. Use Six Star MLS gasket kit or Subaru OEM MLS (11044AA633 for SOHC, 11044AA770 for DOHC). Machine heads to 0.002-inch flatness. Replace timing belt, tensioner, idlers, water pump, thermostat, and valve cover gaskets while apart. Use Subaru Cooling System Conditioner (SOA635071). Total job is 10-14 hours labor.

How to Fix

Have dealer perform official oil consumption test (OCT). If consumption exceeds 1 qt per 3,000 miles, piston ring replacement or short block replacement is warranted. Check coverage under Subaru powertrain warranty extension (8 years/100,000 miles) or Oakes v. Subaru settlement. Use 0W-20 full synthetic oil and monitor levels weekly.

How to Fix

Check oil level frequently (every fuel fill-up). Use high-quality synthetic oil that meets Subaru specifications. Inspect PCV system for proper operation. Monitor for turbo seal leaks. If consumption is excessive, oil consumption test at dealer may warrant warranty coverage.

Owner tips & cautions

• TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
• TipSearch Subaru WRX forums and owner groups for real-world experiences and DIY guides
• TipGet multiple quotes from independent mechanics - dealer prices can be 2-3x higher for the same repair

How to Fix

Always use 93 octane fuel. If tuning, use a reputable tuner with proper air/fuel ratios and timing. Consider forged pistons if building engine. Monitor for knock with an accessport or similar device. Catch early signs through oil analysis showing elevated metals.

Owner tips & cautions

• WarningThis is a high-severity issue - ignoring it can lead to costly repairs or safety concerns. Address it promptly.
• TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes

How to Fix

Perform Subaru's official oil consumption test (OCT) at a dealership. If consumption exceeds 1 qt per 3,000 miles, piston ring replacement is the fix. The Yaeger v. Subaru settlement's 8-year/100,000-mile extension has run out on these model years, so ask the dealer about goodwill coverage under service bulletin 02-157-14R. Short block replacement (Subaru 10103AB830) may be offered for severe cases. Monitor oil level weekly and keep records of oil additions.

How to Fix

Replace both head gaskets with MLS (multi-layer steel) gaskets (Six Star or Subaru OEM MLS 11044AA633). Machine both cylinder heads. Replace timing belt, water pump (21111AA110), thermostat, cam seals, and valve cover gaskets while the engine is apart. Use Subaru Cooling System Conditioner.

How to Fix

Have the dealer perform TSB 11-204-23 (dated 04/25/2023): replace the PCV hose assembly (part 11815AC88D) which includes the new enlarged spring clamp. Covered under California Emissions Performance & Defect Warranty (5yr/50,000mi), Federal Emissions Performance Warranty (2yr/24,000mi), or otherwise the 3yr/36k Basic warranty for qualifying VINs.