P0121 on a Mercedes-Benz: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Mercedes with P0121 usually still drives, but it may enter limp mode with reduced power until the throttle issue is fixed.

What P0121 means on a Mercedes-Benz

On a Mercedes-Benz, P0121 means the throttle position signal doesn't match what the engine computer expects. Mercedes uses an electronic accelerator system (E-gas), so the position sensor lives inside the throttle actuator, and a mismatch usually points to the actuator, its wiring, or the pedal module.

Is P0121 common on Mercedes-Benzs?

P0121 appears on Mercedes models with the E-gas throttle system, including the C-Class, E-Class, and S-Class. The throttle actuator and the wiring to it are the usual trouble spots, and a faulty accelerator pedal module can also set this code.

Commonly reported on: C-Class, E-Class, S-Class, ML.

Symptoms

Check engine light on
Reduced power or limp mode
Rough idle or stalling
Gas pedal feels unresponsive

Common causes

most common: Faulty throttle actuator position sensor
common: Wiring or connector issues in the E-gas harness
common: Faulty accelerator pedal module
less common: Failed throttle actuator motor

How to fix it

Fix	DIY?	Typical cost
Inspect the E-gas wiring and connectors for damage	Yes	$50–150
Test the accelerator pedal module and replace it if needed	Mechanic	$200–450
Replace the throttle actuator assembly	Mechanic	$500–1100