P0121 on a GMC: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A GMC with P0121 usually still drives, but it may idle rough or go into reduced power mode until the throttle issue is fixed.

What P0121 means on a GMC

On a GMC, P0121 means the throttle position sensor is reporting a reading that doesn't match expected engine conditions. Older GMC trucks use a separate TPS on the throttle body, while newer models use a drive-by-wire throttle with the sensor built in.

Is P0121 common on GMCs?

P0121 is common on GMC trucks and SUVs like the Sierra, Yukon, Terrain, and Acadia. On older models the TPS wears out and is easy to replace; on newer ones the throttle body gets dirty or the pedal position sensor fails.

Commonly reported on: Sierra, Yukon, Terrain, Acadia.

Symptoms

Check engine light on
Rough idle or stalling
Poor or surging acceleration
Reduced power or limp mode

Common causes

most common: Worn throttle position sensor
common: Carbon buildup in the throttle body
common: Faulty accelerator pedal position sensor
less common: Damaged wiring or connector at 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	$60–200
Replace the electronic throttle body or pedal sensor	Mechanic	$350–800