P0116 on 2015-2019 Subaru Outback 2.5L: ECT Sensor, Thermostat, and Software Causes

What's Unique About the 2015-2019 Subaru Outback

For this generation of Subaru Outback, the P0116 code has a significant non-hardware cause. Subaru issued Technical Service Bulletin #11-201-21 on Feb 4, 2021, which identifies a software logic flaw in the ECM. This glitch can incorrectly trigger a P0116 code, particularly in cold weather and often when an accessory engine block heater is used. The ECM sees a conflict between the pre-warmed coolant and other cold sensor readings, misinterpreting it as a fault. Additionally, the FB25 engine's cooling system can be difficult to bleed completely, and trapped air pockets can cause erratic sensor readings that mimic a failed part.

Symptoms You May Notice

• Check Engine Light is on
• Temperature gauge behaving erratically (e.g., staying on cold, jumping to hot, or dropping at highway speed).
• Cooling fans running constantly or not turning on when needed.
• Decreased fuel economy.
• Engine running rough or hesitating.
• Difficulty starting the engine, especially in cold weather.
• Heater may not blow warm air.

Most Likely Causes

1. Faulty Engine Thermostat (Stuck Open) 🔴 High Probability → Shop Integrated Thermostat Housing Assembly This is a common failure point on many modern vehicles. If stuck open, the engine fails to warm up at the rate the ECM expects, triggering a performance code. The temperature gauge may also drop towards cold when driving at speed and then rise back up when idling.
   How to confirm: Use a scan tool to monitor live ECT data. If the engine takes an excessively long time to reach operating temperature (e.g., stays below 170-180°F after 15-20 minutes of driving), the thermostat is likely stuck open. The OEM thermostat is rated to open at 180°F (82°C).
   Typical fix: Replace the thermostat and gasket. On the FB25, this is located on the bottom of the engine where the lower radiator hose connects. The system must be drained, refilled, and properly bled of air. It is highly recommended to use an OEM thermostat.
   Est. part cost: $25-$75
2. Faulty Engine Coolant Temperature (ECT) Sensor 🔴 High Probability → Shop Engine Coolant Temperature Sensor The sensor is a thermistor that can degrade over time, leading to incorrect or erratic resistance readings that the ECM interprets as a fault. On the FB25 engine, it is located on the coolant crossover pipe near the top front of the engine, making it relatively easy to access.
   How to confirm: With the engine cold (off for several hours), use a scan tool to compare the ECT reading to the Intake Air Temperature (IAT) reading. They should be within 2-5 degrees of each other. If they differ significantly, the ECT sensor is likely faulty. You can also test the sensor's resistance with a multimeter; at 68°F (20°C), it should be between 2000-3000 ohms.
   Typical fix: Replace the ECT sensor. This usually involves partially draining the coolant, unscrewing the old sensor from the crossover pipe, and quickly installing the new one to minimize coolant loss.
   Est. part cost: $20-$75
3. ECM Software Logic Error 🟡 Medium Probability Subaru TSB #11-201-21 specifically identifies an ECM logic flaw that can trigger P0116, especially in cold weather or when an engine block heater is used. The ECM gets conflicting data between the warm coolant and cold air sensors.
   How to confirm: This cannot be confirmed with simple tools. If the code appears in very cold weather (e.g., -25°F to -40°F) and other hardware checks out, contact a Subaru dealer and reference TSB 11-201-21. The dealer can check your vehicle's Calibration Identification number (CID) to see if the update applies.
   Typical fix: The Subaru dealership needs to perform an ECM software update (reflash).
   Est. part cost: $0-$150 (cost of dealer labor for reprogramming)
4. Low Engine Coolant or Air in System 🟡 Medium Probability → Shop Engine Coolant / Antifreeze If the coolant level is low, the sensor may be exposed to air instead of coolant, causing erratic readings. The FB25 engine is also known to be somewhat difficult to bleed, making air pockets a common issue after a coolant service. This is a frequent cause of persistent P0116 codes after other parts have been replaced.
   How to confirm: Check the coolant level in the overflow reservoir and the radiator (only when the engine is cold). If service was recently performed, suspect air in the system. Look for signs of dried coolant spray in the engine bay, indicating a leak.
   Typical fix: Top off the coolant to the correct level. If air is suspected, the system needs to be properly bled, which involves using a spill-free funnel and elevating the front of the vehicle to help purge air pockets.
   Est. part cost: $15-$30 (for a bottle of Subaru Super Coolant)

Rare But Worth Checking

• Damaged Wiring or Connector to ECT Sensor: → Shop Engine Coolant Temperature Sensor Corrosion or physical damage to the wires or the connector pins can disrupt the signal. It's less common than part failure but should be checked before replacing the sensor.
• Faulty Engine Control Module (ECM): This is extremely rare. The ECM should only be considered after all other possibilities, including the TSB, have been thoroughly ruled out by a professional.
• Faulty Thermo Control Valve (TCV) (2019+ models): → Shop Integrated Thermostat Housing Assembly For the 2019 model year, Subaru introduced a more complex Thermo Control Valve (TCV). This component can fail and produce symptoms identical to a bad ECT sensor or thermostat, including a P0116 code. Subaru has issued an extended warranty for this specific part due to high failure rates.

Diagnosis Steps

1. Check the engine coolant level in the reservoir and radiator (when cold). Top off if low.
2. Use an OBD-II scanner to check for any other codes, particularly P0128, P0117, or P0118.
3. With the engine cold-soaked (off for at least 8 hours), use the scanner's live data function to compare the Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) readings. They should be within 5°F (3°C) of each other. If not, the ECT sensor is likely faulty.
4. If the readings are close, start the engine and monitor the ECT data as it warms up. The temperature should rise steadily. If it rises very slowly, drops at highway speeds, or fails to reach normal operating temperature (approx. 180-210°F) after 15-20 minutes of driving, the thermostat is likely stuck open.
5. If the ECT reading is erratic, jumping around, or stuck at an extreme value (e.g., -40°F or 250°F), suspect a faulty sensor or a wiring issue.
6. Visually inspect the ECT sensor's wiring and connector for any signs of corrosion, damage, or loose connections. The sensor is on the front coolant crossover pipe.
7. If the code appeared in very cold weather (below freezing), especially if you use a block heater, investigate TSB 11-201-21 with a dealer before replacing any parts. One owner reported the code at -40°F.

Parts You'll Likely Need

• Engine Coolant Temperature Sensor (OEM #22630AA280) — This sensor is a primary cause of failure, sending incorrect or erratic signals to the ECM. It is located on the front coolant crossover pipe.
  Trusted brands: Subaru (OEM), Denso, NTK
  OEM price range: $50-$75
  Aftermarket price range: $20-$45
• Engine Coolant Thermostat and Gasket (OEM #21210AA072) — A thermostat stuck in the open position is a very common cause, preventing the engine from warming up correctly. OEM parts are strongly recommended as some aftermarket versions have incorrect specifications.
  Trusted brands: Subaru (OEM), Aisin
  OEM price range: $25-$50
  Aftermarket price range: $15-$35

Related Codes That Often Appear With This One

• P0128 — P0128 (Coolant Thermostat - Coolant Temperature Below Thermostat Regulating Temperature) often appears with P0116 because a thermostat stuck open is a common root cause for both codes.
• P0117 — P0117 (Engine Coolant Temperature Sensor Circuit Low) indicates a short circuit in the sensor's wiring. A wiring fault can cause the irrational readings that trigger P0116.
• P0118 — P0118 (Engine Coolant Temperature Sensor Circuit High) indicates an open circuit. This could be a broken wire or a completely failed sensor, which would also lead to a P0116 performance code.

Technical Service Bulletins (TSBs) & Recalls

• TSB 11-201-21: Addresses an ECM logic flaw that can cause P0116 in cold weather, particularly when an engine block heater is used. The fix is a software update. According to TSB Bulletin #11-201-21, new reprogramming files are available to optimize the ECM and address hard starting in cold temperatures when an accessory engine block heater is utilized.

Platform-Specific Known Issues

• A software update may be required per TSB 11-201-21, especially if the code appears in cold weather or with the use of a block heater.
• The FB25 cooling system can be prone to trapping air, which requires a specific bleeding procedure (e.g., using a spill-free funnel on an incline) to resolve.
• Even after the TSB reflash, one owner on subaruoutback.org reported the code returning, with the final fix being a new thermostat, suggesting the TSB may not solve all cold-weather cases.

Mechanic-Grade Diagnostic Values

• ECT Sensor Resistance — expected: Approx. 2000-3000 Ohms at 20°C (68°F), decreasing to approx. 200-300 Ohms at 90°C (194°F).. Failure: Resistance is outside the expected range at a known temperature, or shows an open/short circuit.
• ECT Sensor Signal Voltage at ECM (Key On, Engine Cold) — expected: Approx. 2.0V - 3.5V.. Failure: Voltage is near 5V (open circuit) or 0V (short circuit).
• ECT Sensor Signal Voltage at ECM (Engine at Operating Temp) — expected: Approx. 0.5V - 1.3V.. Failure: Voltage does not drop as the engine warms up, or remains high (e.g., >1.5V).
• Advanced Bleeding Procedure Step — expected: After filling and running the engine with a spill-free funnel, rev the engine between 2500 to 4000 RPM intermittently for 5 minutes to force air out of the system.. Failure: If warm air does not come from heater vents, air is still trapped in the heater core, and the procedure must be repeated after cooling.

Scan Tool Commands That Help

• Subaru Select Monitor (SSM) or J2534 tool: FlashWrite Reprogramming — This function is required to apply the ECM software update specified in TSB #11-201-21. It is used after confirming the issue is not hardware-related and the vehicle symptoms match the TSB (e.g., P0116 with hard starting in cold when using a block heater).

Wiring & Ground Locations

• ECT Sensor Connector — On the coolant crossover pipe at the top front of the FB25 engine.. This is the primary connection point to check for corrosion, damage, or a loose fit, which can cause erratic or out-of-range signals that trigger P0116.
• ECT Sensor Ground — The ECT sensor grounds through the Engine Control Module (ECM). One of the two wires in its connector is the sensor ground reference provided by the ECM.. A poor ground connection at the ECM or a break in the ground wire can cause the sensor's voltage reading to be skewed high, leading to a P0116 or P0118 code.

Real Owner Repair Stories

• Reddit user on r/subaru (2015 Outback) — Recurrent P0116 code appearing in extreme cold weather (~40 below zero).
  ❌ Tried (didn't work) The user noted this was the second time it had happened, implying a previous fix (or clearing the code) did not permanently resolve it.
  ✅ What actually fixed it The symptoms are a classic match for the issue described in Subaru TSB #11-201-21, which requires an ECM software reflash to correct the logic error that occurs in extreme cold, especially when a block heater is used.
• Aggregated forum reports (2015-2019 Outback 2.5L) — P0116 code returns shortly after replacing the thermostat and/or ECT sensor.
  ❌ Tried (didn't work) Replacing thermostat, Replacing ECT sensor
  ✅ What actually fixed it The cooling system was improperly bled. After performing a thorough bleeding procedure using a spill-free funnel and elevating the front of the vehicle to purge trapped air pockets, the code was permanently resolved.

OEM Part Supersession History

• Unknown → 22630AA280 — This is the current standard OEM part number for the ECT sensor for this vehicle range.
• Unknown → 21210AA072 — This is the current standard OEM part number for the thermostat for this vehicle range.
  Heads up: Using aftermarket thermostats with slightly different opening temperatures can sometimes fail the ECM's rationality check and continue to cause P0116. Sticking to the OEM 82°C/180°F spec is critical.

Model Year Variations Within This Range

• 2019: The 2019 model year introduced the more complex Thermo Control Valve (TCV) as part of the cooling system. This part integrates thermostat functions and can fail, causing P0116. This is a significant departure from the 2015-2018 models which use a traditional, separate thermostat.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Excessive Oil Consumption 🟠 Medium — More common in earlier FB25 engines (2011-2014) which had a warranty extension for new short blocks, but consumption of up to 1 quart per 1200 miles is considered 'normal' by Subaru on later models. (Ref: TSB 02-157-14R (for earlier models))
• Lineartronic CVT Transmission Issues 🔴 High — Issues like shuddering, hesitation, and stalling led Subaru to extend the CVT warranty for many models, including the 2015-2019 Outback, to 10 years / 100,000 miles. (Ref: CVT Warranty Extension Program (WP-72))
• Parasitic Battery Drain 🔴 High → Shop Vehicle Battery — A widespread and well-documented issue where the battery dies after the car sits for a few days. It is often caused by the Data Communication Module (DCM) for the Starlink system failing to enter sleep mode. (Ref: Service Program WRD-24 (for later models))
• Spontaneously Cracking Windshields 🟠 Medium — A high number of owners report windshields cracking easily without impact, leading to a class-action lawsuit for 2015-2016 models. (Ref: TSB 12-192-15 (related to warranty extension))
• Failing Wheel Bearings 🟡 Low — A humming noise at highway speeds is a common symptom of a failing wheel bearing, which is a frequently reported repair for this generation.
• Infotainment System Glitches 🟡 Low — Particularly on 2015-2017 models, the head unit is known for freezing, phantom calls, and unresponsiveness. The system was updated for 2018.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, only used wiring pigtails or connectors are a sensible choice. The primary failure parts (sensor, thermostat) are inexpensive new and are wear items, making used ones a poor value proposition.

Donor-vehicle mileage cap: roughly under 50000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For a wiring connector: ensure locking tabs are intact, no visible corrosion on pins, and at least 6 inches of wire are available to splice.
• Avoid parts from vehicles with signs of major front-end collision or engine fire.

OEM-only on this vehicle (don't cheap out):

• Engine Coolant Thermostat: Strongly recommended to use OEM. The precise 180°F (82°C) opening temperature is critical for the ECM's rationality checks, and aftermarket versions with slight variations can cause the P0116 code to persist.

Aftermarket brands forum-validated for this vehicle:

• Denso (for ECT Sensor)
• NTK (for ECT Sensor)
• Aisin (for Thermostat, if OEM is unavailable)

Brands owners have reported issues with on this vehicle:

• Unnamed, 'white-box', or ultra-low-cost sensors and thermostats from online marketplaces have a high failure rate and are often out-of-spec from the factory, leading to the code returning immediately.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0128 — Often checked alongside P0116; relates to the engine failing to reach operating temperature, frequently caused by a thermostat stuck open.
• P0117 — A companion code indicating low voltage/high temperature from the ECT sensor; can be triggered by actual overheating or sensor failure.
• P0118 — A companion code indicating high voltage/low temperature from the ECT sensor; often checked during P0116 diagnosis to rule out circuit issues.

Frequently Asked Questions