Parts for your 2017 Subaru Legacy-Throttle position sensors

2017 Subaru Legacy throttle position sensors: purpose, care, and when to replace

Drawing on the Subaru Factory Service Manual for the 2017 Legacy/Outback (Engine/Diagnostics sections covering DTCs P0120–P0124 and P2135) and Subaru’s parts catalogue, the 2017 Subaru Legacy does use a throttle position sensor. It’s built into the electronic throttle body as dual sensors, working alongside the accelerator pedal position sensor in the drive‑by‑wire system. That makes the throttle position sensor highly relevant to how this car runs under the bonnet.

On this model, the throttle position sensors (TPS) tell the engine control module exactly how far the throttle plate is open, so it can match air, fuel, and spark for smooth take‑offs, tidy idle, and decent economy. With two sensor tracks for redundancy, the ECU constantly cross‑checks them for accuracy. When they drift or get noisy, you’ll often feel hesitation, erratic idle, cruise control dropping out, or see the check engine light with codes like P0121–P0123 or correlation faults such as P2135. Limp‑home mode isn’t uncommon if the readings don’t agree.

As far as servicing goes, the TPS isn’t a scheduled replacement item, but it benefits from a bit of care. During routine services, a quick scan‑tool sweep of TPS1/TPS2 from closed to wide‑open throttle should show a smooth, linear climb with both signals tracking closely. If the trace is jumpy, flat, or mismatched, it’s worth digging deeper before it turns into an on‑road headache.

Keeping the throttle body clean helps the sensors and motor do their job. Use a throttle‑body‑safe cleaner sparingly with the battery disconnected, don’t force the butterfly by hand, and avoid flooding the electronics. If the gasket looks tired, replace it. After cleaning or replacement, an idle relearn may be required—many shops use a scan tool to run the ETC relearn, though a controlled warm‑up and idle period can also allow the ECU to settle. If a sensor is confirmed faulty, Subaru supplies the TPS integrated with the throttle body, so the fix is to replace the assembly, then clear codes and perform the relearn.

• Typical signs to watch: surging at light throttle, poor fuel consumption, hard starting when warm, and throttle‑related DTCs.
• Good practice: inspect connectors for corrosion, verify pedal sensor data too, and road‑test under gentle and steady throttle to confirm behaviour.

Popular questions

Does the 2017 Subaru Legacy have a separate TPS or is it built in?
It’s built into the electronic throttle body as dual sensors. There’s also a separate accelerator pedal position sensor. The ECU compares both, which is why “correlation” codes can appear if either side misbehaves.

What symptoms point to a bad TPS on a 2017 Legacy?
Common signs include a rough or high idle, hesitation off the mark, cruise control dropping out, and the check engine light with codes like P0121–P0123 or P2135. A scan‑tool graph that doesn’t sweep smoothly from closed to open throttle is another giveaway.

Can the TPS be cleaned, or does the whole throttle body need replacing?
The sensors are sealed into the throttle body, so if the TPS itself is faulty the assembly is replaced. Cleaning the throttle plate and bore can restore smooth operation when the issue is light carbon build‑up rather than a failed sensor.