Parts for your 2017 Subaru Legacy-Temperature sensors

2017 Subaru Legacy temperature sensors — what they do and how to look after them

Temperature sensors are absolutely used on the 2017 Subaru Legacy. Subaru’s own Factory Service Manual for the 2017MY Legacy/Outback details several, including the Engine Coolant Temperature (ECT) sensor and Intake Air Temperature (IAT) sensor within the Engine (Diagnosis/Control) sections, plus an Automatic Transmission Fluid (ATF) temperature sensor in the CVT control system and an ambient air temperature sensor for the HVAC and dash display. These sensors feed the ECU/TCM/BCM for fuelling, ignition timing, fan control and climate regulation. Industry standards such as SAE J1979/ISO 15031 also specify ECT as a mandated OBD‑II parameter, which aligns with Subaru’s diagnostic trouble codes (for example, P0115–P0119 series for ECT circuit and range/performance). So yes—temperature sensors are relevant, fitted, and central to how a 2017 Legacy runs.

On this model, the ECT sensor is the big decision-maker for warm‑up enrichment, radiator fan operation and over‑temperature protection under the bonnet. The IAT sensor helps fine‑tune mixture and spark, particularly on hot days or cold starts. The CVT’s fluid temperature sensor looks after shift strategy and transmission protection, especially towing or in alpine conditions. The ambient sensor informs the climate control and the outside temp readout, keeping cabin comfort steady without driver fuss. When these sensors read accurately, the car starts crisply, uses fuel efficiently and keeps its cool in Aussie and Kiwi summers.

There’s no fixed replacement interval for these sensors, but they should be checked any time a cooling system service or drivability diagnosis is on the cards. Good servicing practice includes:

• Scan-tool checks from cold start: ECT should read close to ambient and rise smoothly.
• Inspect connectors and looms for corrosion, oil or coolant wicking at the plug.
• Maintain coolant quality, old or contaminated coolant can shorten ECT sensor life.
• For CVT, keep to fluid service recommendations if applicable—heat is the enemy.

Replacement is straightforward for most DIYers: allow the engine to cool, depressurise the system, disconnect the battery, unplug the sensor and swap it with an OEM‑quality part and a fresh seal. Top up with the correct Subaru long‑life coolant mix and bleed air properly. After refitting, verify readings with a scan tool and confirm radiator fan operation. If the check engine light appears, hard starts, rich running, erratic fans, or a wildly inaccurate outside temperature pop up, it’s time for testing and likely a new sensor.

Popular questions

Where are the temperature sensors on a 2017 Subaru Legacy?

The ECT sensor is threaded into the engine’s coolant passage near the thermostat housing, under the bonnet. The IAT is integrated with the intake tract (often within or near the MAF housing). The CVT fluid temp sensor sits inside the transmission and is not typically serviced externally. The ambient sensor is mounted ahead of the radiator/condenser area, exposed to airflow.

What are the symptoms of a failing coolant temperature sensor?

Common signs include hard cold starts, high idle, black smoke or rich fuel trims, radiator fans running constantly or not at all, poor economy and a check engine light with ECT‑related codes. Scan data that’s stuck at a fixed temperature or jumps around is another giveaway.

Do temperature sensors need routine replacement, and what might it cost?

They’re not a scheduled replacement item, but age, heat and coolant condition can take their toll. In Australia or New Zealand, an ECT sensor and seal from a reputable brand is typically modest in cost, with labour adding a small amount if coolant bleeding is required. Expect higher costs if diagnosis time or additional cooling service is needed.