P0117 on Subaru: How to Diagnose & Fix
Engine Coolant Temperature Sensor Circuit Low
1 Subaru model · 1 documented issue · 1 critical · $1,800-$3,500 repair
P0117 on Subaru by model: symptoms, diagnosis and fix
The EJ25 2.5L horizontally-opposed engine in 1998-2010 Foresters is notorious for external head gasket failure. Unlike typical blown head gaskets that cause overheating, the Subaru EJ25 gaskets fail externally, leaking coolant and oil down the sides of the engine. The failure is caused by the gasket material (composite) breaking down over time, combined with the horizontal orientation placing uneven stress on the gaskets. This is widely considered the most common and well-documented Subaru defect.
Common Symptoms
- Sweet coolant smell from engine bay
- Oil weeping down sides of engine block
- Coolant level dropping with no visible external puddle
- White residue or staining on lower engine block
- Overheating in severe cases where internal gasket failure occurs
How to Fix
Replace both head gaskets with the updated MLS (multi-layer steel) gaskets. Resurface both cylinder heads to ensure flatness within spec (0.002" max). Replace thermostat, water pump, timing belt, and all idler/tensioner pulleys while the engine is apart — these are all accessible during the job. Use genuine Subaru coolant (SOA868V9210). Budget 10-14 hours labor for a boxer engine head gasket job.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Subaru 11044AA633 MLS Cylinder Head Gasket
head-gasket component after teardown and machine-shop inspection confirms this exact branch Fitment/availability: 1999-2010 Forester 2.5L SOHC naturally aspirated branch only; quantity two, confirm VIN and engine Fitment/availability: sh…
- Genuine Subaru 21200AA072 Thermostat
cooling-system service component when replacement is indicated during the verified repair Fitment/availability: Retailer-cataloged 1998-2010 Forester EJ25 applications in this issue; confirm VIN and order the correct seal separately Fitm…
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Sources for this issue (3)
P0117 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0117 mean on a Subaru?
P0117 stands for "Engine Coolant Temperature Sensor Circuit Low." This code means the engine coolant temperature (ECT) sensor is reading too low a voltage, which the computer interprets as an impossibly high coolant temperature. In most cases this is an electrical fault — such as a short to ground in the wiring — rather than the engine actually overheating. The coolant temperature signal affects fuel mixture, cooling fans, and other functions, so a bad reading can cause poor running, hard starting, or fans running constantly. The fault is usually in the sensor or its wiring. On Subaru it is documented across 1 model: Forester.
What are the most common causes of P0117?
The most common causes of P0117 are: Faulty engine coolant temperature sensor, Short to ground in the sensor signal wire, Damaged or corroded wiring/connector at the sensor, Coolant contamination affecting the sensor, Poor electrical ground, Faulty PCM/ECM (less common). The specific cause varies by vehicle.
How much does it cost to fix P0117 on a Subaru?
Published repair estimates on Subaru range from $1,800 to $3,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0117 on a different make?
View P0117 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.