P0116 Code: Subaru Forester (2019-2024) - Causes, Symptoms & Fixes

CODE MEANING AND SEVERITY

• Code: P0116
• Full name: Engine Coolant Temperature Sensor Range/Performance (CIRCUIT)
• What it means: The Engine Control Module (ECM/ECU) has detected that the Engine Coolant Temperature (ECT) sensor reading is out of its expected range or is not changing properly with temperature. In practice, the ECM may see the coolant temperature as either too cold or too hot, or the sensor signal may not respond to temperature changes as it should.
• Severity for a 2019-2024 Subaru Forester:
  • Moderate to potentially serious. The MIL (check engine light) typically comes on, and the PCM may switch to open-loop fueling or default strategies, which can affect fuel economy and drivability.
  • If the symptom progresses toward actual overheating (rare with just a sensor range fault, but possible if the sensor misleads the PCM about true coolant temp), there is a risk of engine damage. Monitor coolant temperature gauge and engine temperature closely.
• Data note: No recalls found in the NHTSA database for this specific model/years/issue, and no official owner complaints documented in the provided data. This guide uses general automotive knowledge to explain P0116, along with Forester-specific considerations.

COMMON CAUSES ON SUBARU FORESTER

• Faulty Engine Coolant Temperature (ECT) sensor
  • Most common failure mode for P0116.
  • Sensor can drift, stick, or read out of range.
• Damaged or corroded wiring/connector to the ECT sensor
  • Pinched wires, chafing near the engine, or poor/oxidized connections can cause erroneous readings.
• Ground or power/five-volt reference issues
  • A bad ground or a faulty reference line can produce incorrect sensor signals.
• Faulty thermostat or cooling system problems
  • If the thermostat sticks closed or to an incorrect position, the coolant temperature seen by the sensor can be inconsistent with actual temperature (or the PCM can misinterpret a sensor that’s not responding to temperature change).
• Coolant level or condition issues
  • Low coolant, contaminated coolant, air in the cooling system, or a leak can affect sensor readings indirectly through flow and sensor reference conditions.
• PCM/ECM concerns (less common)
  • A malfunctioning PCM could misinterpret valid sensor data, though this is far less likely than a sensor or wiring fault.
• Forester-specific considerations
  • Subarus tend to present P0116 as ECT sensor or harness issues first; the sensor is normally located at or near the thermostat housing. Cooling system maintenance (coolant quality, bleed/bubble-free fill) is important in keeping sensors reading correctly.

SYMPTOMS

• Check Engine Light (MIL) on or flashing
• Inconsistent or misleading temperature display on the dash (gauge may read unusually cold or hot)
• Reduced or erratic engine performance, especially at startup or during idle
• Poor fuel economy or lean/rich air-fuel trim indications from a scan tool
• Occasional rough idle or misfire-like symptoms if fueling is affected
• Heater performance issues (e.g., heater not getting warm quickly) can accompany incorrect coolant readings
• In rare cases, if the ECU detects an extreme reading, it may enter a limp or degraded-performance mode to protect the engine

DIAGNOSTIC STEPS

Note: Begin with a scan to confirm P0116 and then verify with live data. Use a reliable OBD-II scanner that can read live PID data and freeze-frame data.

• Step 1: Verify code and reset

  • Read DTCs with ignition on, engine off, then start the engine and re-check to see if P0116 returns.
  • Record freeze-frame data (engine temp, Coolant Temperature Sensor value, engine RPM, etc.).
  • Clear codes and drive normally to see if the code returns.

• Step 2: Visual inspection

  • Inspect the ECT sensor connector and wiring for corrosion, damaged insulation, or loose pins.
  • Check the coolant level and condition. Look for leaks around the radiator, hoses, and thermostat housing.
  • Check for air in the cooling system; if recently serviced, ensure proper bleeding/air removal.

• Step 3: Compare live data (sensor vs. actual temperature)

  • Using a live data stream, compare the ECT sensor reading to real coolant temperature (use a diagnostic infrared thermometer or known coolant temperature in the engine at startup).
  • At cold start, ECT should read low (close to ambient) and rise as the engine warms. If the ECT value does not change with actual coolant temperature, suspect sensor or circuit fault.
  • If the dash gauge or cluster shows a temperature that disagrees with the ECT sensor value, rely on the sensor data for diagnosis and investigate the gauge circuit if needed.

• Step 4: ECT sensor resistance/voltage test (DC volt/ ohms test)

  • With the engine cold, unplug the ECT sensor and measure resistance across the sensor terminals (or measure the sensor’s reference voltage and signal voltage with the PCM powered).
  • Reconnect and gradually heat the engine to normal operating temperature and observe if the sensor value changes smoothly.
  • If resistance values do not change with temperature or readings are wildly out of spec, replace the ECT sensor.
  • Important: consult the Subaru service manual for the correct resistance vs. temperature table and reference voltages for your exact engine code (FA/FB family). If you don’t have the spec, replacing the sensor is a reasonable next step after confirming the circuit is intact.

• Step 5: Examine wiring and connectors

  • Check for continuity between the PCM side and the sensor connector.
  • Inspect for damaged insulation, exposed wires, or corrosion at the connector pins.
  • Confirm the 5V reference line and ground path are intact; a fault on the reference line can cause erroneous readings.
  • If a problem is found in wiring, repair or replace the affected harness segment and re-test.

• Step 6: Thermostat and cooling system check

  • If the sensor itself tests good but readings still appear wrong, inspect the thermostat operation (stuck open/closed), radiator fan operation, and overall cooling system performance.
  • Ensure proper coolant mixture (Subaru typically uses a 50/50 mix with Subaru-approved coolant) and perform a coolant bleed as needed.
  • Consider a coolant flush if the coolant is degraded or contaminated.

• Step 7: Additional testing if needed

  • If the ECT circuit tests good and the readings still don’t make sense, consider testing other engine temperature-related sensors and the PCM logs to see if there is a broader signal processing issue.
  • If all electrical checks are clean and readings still misbehave, PCM replacement is a last-resort consideration after dealer diagnostic confirmation.

RELATED CODES

• P0115 = Engine Coolant Temperature Sensor Circuit Malfunction
• P0117 = Engine Coolant Temperature Sensor Circuit Low Input
• P0118 = Engine Coolant Temperature Sensor Circuit High Input
• P0128 (not the same code family, but related to thermostat/Coolant temperature behavior) may appear in some cases where the thermostat or temperature readings are part of the fault.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Prices are rough ranges based on typical U.S. market pricing. Actual costs depend on your region, shop rates, and whether OEM or aftermarket parts are used.

• ECT sensor replacement

  • Parts: $15–$60
  • Labor: $40–$120
  • Total: approximately $55–$180
  • Notes: The ECT sensor on Forester is usually located near the thermostat housing and is among the simpler repairs.

• Thermostat replacement (if thermostat is the root cause or contributing factor)

  • Parts: $10–$60
  • Labor: $120–$350
  • Total: roughly $130–$410
  • Notes: If the thermostat is stuck or not opening properly, replacing it is advisable along with cooling system inspection.

• Coolant flush and refill

  • Parts (coolant): $8–$25
  • Labor: $80–$150
  • Total: $90–$175
  • Notes: Use Subaru-approved coolant and correct mix ratio.

• Wiring harness/connector repair

  • Parts: $5–$40 (connector or harness section)
  • Labor: $60–$180
  • Total: $65–$220
  • Notes: If the fault is in the wiring or connector, repairs can be modest or moderate depending on damage.

• PCM/ECM-related replacement (rare)

  • Parts: $400–$1200 (plus a service/diagnostic visit)
  • Labor: $100–$300
  • Total: $500–$1500+
  • Notes: Only consider if a fault cannot be found in the sensor or wiring after extensive testing.

• Preventive service if multiple cooling-system components are due

  • Combined services (coolant flush, hose inspection, sensor inspection, thermostat check) may run $200–$600 depending on scope and labor.

DIY VS PROFESSIONAL

• DIY (potentially suitable for confident, tool-equipped enthusiasts)

  • Pros: Lower cost, hands-on learning, faster turnaround if you already own tools.
  • Cons: Working around the cooling system and ECT sensor requires care to avoid burns and coolant spills; risk of incorrect reassembly leading to leaks or cooling issues.
  • What to do yourself:
    • Inspect hoses and the ECT sensor connector.
    • Use a basic scan tool to verify live data and compare ECT vs actual coolant temp.
    • If comfortable, perform the sensor resistance check and wiring continuity tests with a multimeter.
    • If a fault is found in the sensor or wiring, consider replacement and re-test.

• Professional (recommended if you lack tools or want a definitive diagnosis)

  • Pros: Proper diagnostic procedure, pressure/cooling-system testing, precise sensor testing, and warranty on repairs.
  • Cons: Higher upfront cost, but often faster and safer.

• When to choose professional service

  • If the code returns after replacement of the ECT sensor, or if you discover wiring harness damage, corrosion, or a suspected PCM fault.
  • If cooling-system issues are present (leaks, overheating) or you’re not comfortable with bleeding the cooling system.

PREVENTION

• Maintain proper coolant level and concentration
  • Use Subaru-approved coolant, typically a 50/50 mix, and replace according to Subaru maintenance guidelines.
• Regular cooling-system maintenance
  • Inspect hoses, radiator, clamps, and the radiator cap. Replace leaking or degraded hoses.
  • Periodically flush the cooling system as recommended by Subaru for your model year and engine.
• Ensure proper thermostat operation
  • If there are signs of thermostat issues (slow warm-up, fans running excessively, or overheating), address promptly.
• Protect electrical connections
  • Inspect the ECT sensor connector for corrosion or loose pins; keep connectors clean and dry. Use dielectric grease on connectors if appropriate.
• Catch contaminants that affect readings
  • Ensure no oil or coolant contamination around the sensor area. A leak or spill can affect sensor readings.
• Drive habits and maintenance
  • Regularly scan for DTCs and address P codes early to prevent extended drivability issues.
  • Don’t ignore engine temperature gauge anomalies; they can indicate sensor or cooling-system problems that may escalate over time.

Data limitations and transparency

• The provided data states: No recalls found in the NHTSA database for the 2019-2024 Subaru Forester related to P0116, and no owner complaints were listed for this exact issue. This guide uses general automotive knowledge and typical P0116 symptoms/repairs, tailored for Subaru Forester variants in this year range.
• Real-world variations exist between vehicles; if your Forester has a specialized engine variant (e.g., different FA/FB engine family), refer to the official service manual for exact sensor resistance values and specific service procedures.
• If you suspect a fault beyond ECT sensor and wiring (such as PCM issues or multiple coolant-system faults), a professional diagnostic approach with OEM-level tooling is advised.