Parts for your 2018 Subaru Impreza-Temperature sensors

2018 Subaru Impreza temperature sensors: what they do and how to look after them

Based on the Subaru factory service manual for the FB20 engine (2017–2019 Impreza), Subaru TechInfo wiring diagrams, Denso OE application guides, and SAE OBD-II documentation (J1979/J2012), the 2018 Subaru Impreza absolutely uses multiple temperature sensors. They’re a core part of how the ECU and control modules manage cold starts, fueling, emissions, cooling fans, and the CVT’s behaviour.

On this model you’ll find several key temperature sensors doing specific jobs:

• Engine Coolant Temperature (ECT) sensor – tells the ECU how hot the engine is so it can adjust fueling, ignition timing, idle speed, and radiator fan control.
• Intake Air Temperature (IAT) sensor – integrated into the MAF, helps with air density calculations for accurate mixture control.
• Ambient air temperature sensor – feeds the HVAC and dash display and can influence fan and A/C logic.
• CVT fluid temperature sensor – inside the transmission, used by the TCM to protect the CVT and manage shift logic.

There’s no routine replacement interval for these sensors, they’re serviced on condition. If the car is hard to start cold, idles oddly, chews through fuel, or the fans run constantly, scan it. Look at live data and check for codes like P0117/P0118 or P0128 (ECT), P0113 (IAT), P0072/P0073 (ambient), and P0711 (CVT fluid temp). Compare ECT and IAT to the actual ambient when the car’s been sitting overnight, they should be within a few degrees of each other.

When replacing an ECT sensor, work on a cold engine, relieve system pressure, and catch the coolant. Swap the sensor and seal as a set, avoid thread tape unless the service manual specifies it, and refill with the correct Subaru long-life coolant. Bleed the cooling system properly to avoid air pockets. A bit of dielectric grease on the connector helps keep moisture out.

For the IAT (in the MAF), only use proper MAF cleaner—no throttle body or brake cleaner—and let it dry fully before refitting. The ambient sensor sits behind the grille and can get knocked in minor front-end bumps, make sure it’s mounted correctly and the wiring isn’t chafed. The CVT fluid temp sensor is internal and not a DIY replaceable item, looking after it is about keeping the CVT fluid fresh to the logbook schedule and ensuring the cooling system is healthy.

Sticking with genuine or OE-equivalent (often Denso) parts keeps readings accurate, which pays off in smooth running, good economy, and long engine and transmission life.

FAQs

Where is the engine coolant temperature sensor on a 2018 Subaru Impreza?
On the FB20 engine it’s threaded into a coolant passage near the thermostat/water outlet at the front of the engine. It’s accessible from above on most cars once the intake snorkel is out of the way. Always disconnect the battery and work on a cold engine before removal.

Do temperature sensors need regular replacement?
No, they’re not a routine service item. Replace them if they’re out of spec or causing drivability issues or codes. Keeping the cooling system maintained, using the correct coolant, and cleaning the MAF/IAT with the right spray helps sensors live a long life.

What fault codes point to a bad temperature sensor?
Common ones are P0117/P0118 and P0128 for coolant temp, P0113 for intake air temp, P0072/P0073 for ambient air temp, and P0711 for CVT fluid temp. Use live data to confirm before replacing parts.