Parts for your 2019 Subaru Legacy-Temperature sensors

2019 Subaru Legacy temperature-sensors

Temperature sensors are absolutely fitted to the 2019 Subaru Legacy and they’re central to how the car runs and protects itself. Subaru’s factory service information (STIS, 2019 Legacy/Outback—Engine Diagnostics) specifies an Engine Coolant Temperature (ECT) sensor, with related OBD-II fault codes like P0117/P0118 for circuit faults and P0128 for thermostat/temperature regulation. The Owner’s Manual references coolant temperature warnings, and the vehicle supports standard OBD-II coolant temperature data (SAE J1979). Beyond the ECT, the Legacy also uses intake air, ambient/outside air, aircon evaporator and (on CVT models) transmission fluid temperature sensors to keep everything sweet under the bonnet and inside the cabin.

What do they all do? The ECT sensor tells the engine computer how warm the motor is so it can adjust fuelling, ignition timing and idle speed, switch radiator fans, manage the aircon load and display temperature warnings. Intake and ambient temp sensors help with cold-start behaviour and fuel trims, while the CVT’s temperature data protects the transmission and smooths shift strategy. In short, the Legacy relies on temperature feedback for efficiency, performance and longevity.

They’re not a routine replacement item, but they do deserve attention as part of normal servicing. Good practice under Aussie and Kiwi conditions includes: checking live data with a scan tool (cold ECT should roughly match outside temp), inspecting sensor plugs and harnesses for corrosion or broken clips, and keeping coolant fresh and at the correct concentration as per Subaru’s long-life coolant schedule. Telltales of a crook ECT sensor include hard cold starts, black smoke, fans running constantly, poor fuel economy, a high idle that won’t settle or a temp warning despite normal coolant level.

If replacement’s on the cards, a technician will usually work on a cool engine, relieve system pressure, drain a litre or two of coolant, unplug the sensor and swap it with a quality OEM-equivalent, renewing any sealing washer or O-ring. It’s then torqued to the Subaru spec, the system is refilled with the correct premix and bled of air with the heater on. After that, they’ll clear codes, verify the reading against ambient and road-test. Pairing sensor replacement with a scheduled coolant service is a neat way to save time and keep the Legacy happy for the long haul.

• Common ECT fault codes: P0117, P0118, P0128
• Symptoms: rich running, rough cold start, fans stuck on, temp warnings
• Quick check: compare cold ECT reading to outside temperature

FAQs

Where is the engine coolant temperature sensor on a 2019 Subaru Legacy?
On the 2.5‑litre FB25, it’s mounted on the coolant outlet/crossover near the thermostat housing at the front of the engine, beneath the intake area. On the 3.6R six-cylinder, it’s located near the upper radiator hose outlet. Access can be tight, so most people tackle it with the right socket and a cool engine.

Do temperature sensors need regular replacement?
Not usually. They’re designed to last and are replaced when faulty. What does help is keeping the cooling system in top nick—fresh long-life coolant, clean connectors and sound wiring reduce the chance of false readings and nuisance codes.

What fault codes point to a bad temperature sensor?
P0117 and P0118 flag ECT circuit low/high inputs, while P0128 suggests the engine isn’t reaching proper operating temp (often a thermostat issue, but the ECT reading should be checked). If the fans are blasting on a cold start or the gauge/light misbehaves, scan it and compare cold ECT to ambient.