Parts for your 2009 Subaru Impreza-Temperature sensors

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

Temperature sensors absolutely are fitted to the 2009 Subaru Impreza and they’re central to how the car runs. Technical references including the Subaru Factory Service Manual for the 2009 Impreza (GE/GH/GR), Subaru’s parts catalogue, and standard OBD‑II data sets confirm multiple temperature sensors: an engine coolant temperature (ECT) sensor, an intake air temperature (IAT) sensor, an ambient/outside air temperature sensor for HVAC/cluster display, and a transmission fluid temperature sensor on automatic models.

The ECT sensor is the big one. It tells the engine control module how hot the engine really is so it can sort cold starts, fuel enrichment, ignition timing, idle speed, radiator fan operation, and overheat protection. That single reading affects driveability, emissions, fuel economy, and engine life. The IAT sensor helps the ECU fine‑tune fuel delivery based on air density, while the ambient sensor feeds the climate control and outside-temp display. On autos, the trans temp sensor manages shift strategy and protects the gearbox when things get toasty.

Typical warning signs of a dodgy temp sensor include hard cold starts, high idle, rich running, poor fuel economy, radiator fans stuck on or never coming on, wonky temp gauge behaviour, and fault codes such as P0115–P0119 or P0128. Left unchecked, incorrect readings can lead to carbon build-up, catalyst damage, or overheating.

• Service checks: During routine servicing, a quick scan-tool check of live data is gold. After an overnight cold soak, ECT and IAT should match ambient. Inspect connectors for corrosion, brittle wiring, or coolant contamination around the ECT.
• Cooling system health: Old coolant, sticky thermostats, and air pockets can make good sensors look bad. Keep coolant fresh to spec and fix leaks promptly.
• When to replace: There’s no set interval, replace if readings are implausible, intermittent, or out of spec when compared to a thermometer at the housing. Many are replaced simply due to age or heat-cycling.
• Replacement tips: Work on a cool engine, relieve pressure, and drain a little coolant. Swap the ECT, fit the correct seal/O‑ring or thread sealant as specified by Subaru, and tighten to the factory torque. Refill and bleed the cooling system, then confirm fan operation and stable temps on a road test.
• Parts choice: Use quality OEM-equivalent sensors. Cheap copies can drift, causing repeat faults.

Done right, it’s an inexpensive, half‑hour sort of job that restores smooth running and peace of mind on any 2009 Impreza, from city runabout to weekend canyon carver.

Popular questions about 2009 Subaru Impreza temperature sensors

Where is the engine coolant temperature (ECT) sensor located?
On EJ-powered 2009 Imprezas, the ECT sensor is threaded into the coolant crossover passage near the top front of the engine. It’s accessible from above with the intake snorkel out of the way. Always work on a cool engine and expect a small coolant loss when removing it.

Can I drive with a bad temperature sensor?
It might still run, but it’s not a good idea. A faulty ECT can cause rich fuelling, poor economy, and fan misbehaviour, which risks overheating. If a scan shows implausible readings or you’ve got temp‑related fault codes, fix it before racking up more kilometres.

Do turbo models have extra temperature sensors?
All models use ECT and IAT. Some markets and earlier generations had additional exhaust temperature monitoring, but many 2009 WRX/STI variants don’t use a dedicated EGT sensor for ECU control. If unsure, check the vehicle by VIN against the Subaru service manual and wiring diagrams.