P0120

What the Code Means

Code P0120 points to a fault in the throttle position sensor (TPS) circuit or the accelerator pedal position sensor circuit. The engine control unit (DME) detects an incorrect signal from one of these sensors, which breaks the correlation between pedal position and actual throttle opening. On N55 engines, Valvetronic can partially compensate for the fault by managing valve lift, but it doesn't fully resolve the problem.

Affected components: throttle position sensor (built into the throttle body), accelerator pedal position sensor (on the pedal module), and the wiring and connectors between them and the DME.

Symptoms

• Engine: unstable idle, surging under acceleration, delayed response to the gas pedal
• Behavior: reduced power, entering limp mode
• Indicators: Check Engine Light (CEL) stays on

Diagnosis and Repair

Step 1: Check the Wiring and Connectors

• Inspect the wiring harness from the throttle and pedal sensors for damage, corrosion, or loose contacts.
• Measure resistance and circuit continuity with a multimeter.

Step 2: Clean the Throttle Body

• Remove the intake boot and clean the throttle body with a dedicated cleaner. Carbon buildup can cause incorrect TPS readings.

Step 3: Replace Faulty Components

• If cleaning doesn't help, replace the throttle body (OEM part number 13547597714).
• If the accelerator pedal is suspected, replace the pedal module.

Step 4: Throttle Adaptation

• After replacement, run a throttle adaptation with ISTA or INPA diagnostic software.

Common Diagnostic Mistakes

• Don't confuse a TPS fault with Valvetronic problems — the symptoms can look similar.

Secondary Problems

Ignoring code P0120 can lead to:

• The engine dropping into limp mode, which significantly reduces handling and driving safety.
• Accelerated wear on other intake system components from an incorrect air/fuel ratio.
• Possible stalling at idle or during a sudden throttle lift.

Timely diagnosis and repair prevent more serious damage and keep the engine running properly.