P0121 on a Subaru: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Subaru with P0121 usually still runs, but the idle may be rough and the throttle may feel unresponsive until it's fixed.

What P0121 means on a Subaru

On a Subaru, P0121 means the throttle position sensor's reading is out of the expected range. Older Subarus use a separate TPS bolted to the throttle body, while newer ones have the sensor built into a drive-by-wire throttle body.

Is P0121 common on Subarus?

P0121 is common on Subarus, especially the Impreza, Outback, and Forester. On older models the separate TPS wears out, and on newer ones a dirty throttle plate or the throttle body assembly is usually at fault.

Commonly reported on: Impreza, Outback, Forester, Legacy.

Symptoms

Check engine light on
Rough idle or stalling
Poor throttle response
Reduced power or limp mode

Common causes

most common: Worn throttle position sensor
common: Carbon buildup on the throttle plate
common: Damaged TPS wiring or connector
less common: Vacuum leak near the throttle body

How to fix it

Fix	DIY?	Typical cost
Clean the throttle body and reset the idle	Yes	$50–180
Replace the throttle position sensor	Yes	$80–220
Replace the drive-by-wire throttle body assembly	Mechanic	$350–750