Parts for your 2018 Subaru Impreza-Throttle position sensors

2018 Subaru Impreza throttle position sensor (TPS): purpose, service tips and replacement

Technical sources for the 2018 Subaru Impreza (FB20 engine) confirm it does use a throttle position sensor—actually a pair—integrated into the electronic throttle body. Subaru’s 2018MY Impreza service manual (Engine/ECM and Electronic Throttle Control sections), the OEM wiring diagrams, and DTC listings (P0121–P0123, P2135) all describe dual throttle position signals and accelerator pedal position (APP) sensors feeding the ECU. The Subaru parts catalogue also lists the TPS as part of the throttle body assembly rather than a separate, serviceable sensor. So yes, the vehicle is fitted with TPS, and it’s integral to drive-by-wire operation.

On the 2018 Impreza, the TPS tells the ECU exactly how far the throttle plate is open. Because it’s a modern drive-by-wire setup, the ECU uses inputs from the accelerator pedal (APP) and compares them with the throttle plate position (TPS). That way it can deliver smooth throttle response, optimise fuel and spark, and keep emissions in check. The throttle body carries two TPS tracks for redundancy, if the signals don’t agree, the car will light up the MIL and likely drop into limp mode to protect the engine.

As part of routine servicing, there isn’t a separate TPS filter or interval—because the sensor is built into the throttle body. What does help is keeping the throttle bore clean and ensuring good electrical health. Under the bonnet, inspect the throttle body connector for corrosion or chafed wiring, and keep the battery and grounds in good nick. If cleaning the throttle, use throttle-body safe cleaner, don’t force the plate by hand, and let the ECU complete an idle relearn afterwards (via Subaru Select Monitor or equivalent, some relearns can occur automatically after a warm-up and steady idle).

If there are symptoms—erratic idle, hesitation, poor fuel economy, or a MIL with codes like P0121, P0122, P0123, or P2135—start with scan data. Check throttle angle versus accelerator pedal angle, they should track smoothly without drop-outs. Wiggle-test the harness and look for vacuum leaks before condemning parts. If the TPS itself is faulty, Subaru supplies it as a complete throttle body assembly. Replacement is straightforward for a competent tech: depressurise as required, disconnect the battery, remove intake ducting, unplug the connector, swap the throttle body with a new gasket/O-ring, torque to spec per the service manual, then perform the idle/throttle relearn. Using quality parts and the correct relearn prevents dramas like high idle or surging afterward.

• Watch for moisture or green crusties in the connector—common cause of intermittent TPS codes.
• If the battery’s been disconnected, expect a brief relearn period, avoid heavy throttle until idle stabilises.

Does the 2018 Subaru Impreza actually have a TPS?

It does. The TPS is integrated into the electronic throttle body and provides two position signals. The ECU compares these with the accelerator pedal sensors to control the throttle plate precisely and safely.

Can the TPS be replaced on its own, or is it part of the throttle body?

On this model it’s part of the throttle body assembly. If the sensor fails, the usual repair is to replace the complete throttle body, fit a new gasket, and carry out the proper idle/throttle relearn with a scan tool.

What symptoms or codes point to a TPS issue on a 2018 Impreza?

Common signs are surging, flat spots, stalling at idle, poor economy and a MIL. Typical codes include P0121–P0123 (TPS circuit range/performance/low/high) and P2135 (throttle/pedal correlation). Always check wiring, connectors and throttle body cleanliness before replacement.