Parts for your 2009 Subaru Forester-Temperature sensors

2009 Subaru Forester temperature-sensors: what they do and how to look after them

Temperature-sensors are absolutely used on the 2009 Subaru Forester (SH). Factory sources confirm multiple temp sensors are fitted: the Engine Coolant Temperature (ECT) sensor for engine management, the Intake Air Temperature (IAT) sensor integrated in the Mass Air Flow (MAF) sensor on most petrol models, an ambient/outside air sensor for the HVAC and display, and an Automatic Transmission Fluid (ATF) temperature sensor on autos. Diesel variants add exhaust gas temperature (EGT) sensors as part of the DPF system. This layout is documented in the Subaru Forester (SH) 2009 Factory Service Manual, Subaru’s technical wiring diagrams, and OEM parts catalogues.

On a 2009 Forester, temperature-sensors quietly keep everything sweet under the bonnet. The ECT sensor tells the ECU how warm the engine is so it can sort cold starts, ignition timing, fuel trims, radiator fan operation, and even A/C logic. The IAT sensor helps the ECU fine‑tune fuel delivery as air density changes. The ambient temp sensor feeds the climate control so it doesn’t blow hot when you want cool. Autos rely on ATF temp data to protect the transmission when towing or climbing in summer heat. If it’s the diesel, EGT sensors keep the DPF’s regeneration in the safe zone.

As part of regular servicing, a workshop should:

• Scan for stored codes and check live data (ECT should climb smoothly from cold to operating temp, IAT should be close to ambient on a cold start).
• Inspect connectors and looms for corrosion, oil soak, or brittle insulation—common around the throttle body and radiator area.
• Confirm radiator fans cycle correctly and that the thermostat isn’t masking a lazy sensor.

Replacement is straightforward for most sensors. On EJ25 petrol models, the ECT sensor sits on the coolant crossover near the intake, expect a small coolant loss, so have fresh coolant ready and bleed the system properly. The IAT is part of the MAF—if IAT readings are off, check for intake leaks first, only replace the MAF when tests confirm it. Ambient temp sensors live ahead of the condenser, if front-end damage has occurred, inspect that bracket and plug. For autos, ATF temp issues often trace to internal transmission harnesses—diagnose before replacing parts. Diesel EGT sensors can seize, use penetrating oil and the correct socket to avoid snapped threads.

Signs of trouble include hard cold starts, high idle, rich running, poor economy, lazy radiator fans, erratic A/C, or the temp gauge acting odd. A quick scan and a few targeted voltage/ohms checks usually pin it down, saving guesswork and keeping the Forester happy on Kiwi and Aussie roads.

Popular questions about 2009 Subaru Forester temperature-sensors

Where is the coolant temperature sensor located on a 2009 Forester?
On most 2.5‑litre petrol models, it’s threaded into the coolant crossover near the top of the engine, close to the throttle body. Access is from above after removing the engine cover. Turbo layouts are similar but tighter around the intercooler. Diesel models use different sensors and additional EGT probes along the exhaust.

What symptoms point to a bad temperature sensor?
Expect cold‑start fussiness, high fuel use, rough idle, radiator fans running at odd times, or a check engine light with ECT/IAT codes. The gauge may behave normally because the cluster uses ECU data, rely on scan-tool live data to verify the sensor is reading plausibly from cold to hot.

Do new sensors need coding or relearning?
No coding is required for ECT, IAT/MAF, or ambient sensors. Clear codes, verify live data, and for coolant work, bleed the system properly. Transmission or diesel exhaust sensors may need a reset of learned values or DPF data using a capable scan tool after repair.