P0120 Throttle Position Sensor/Switch "A" Circuit Malfunction

on

P0120 Throttle Position Sensor/Switch "A" Circuit Malfunction

What Does Code P0120 Mean?

Diagnostic Trouble Code P0120 indicates that the Engine Control Module (ECM/PCM) has detected a malfunction in the Throttle Position Sensor (TPS) "A" circuit. The ECM receives an incorrect or missing voltage signal from the TPS, preventing accurate throttle angle calculation. This fault may result from a defective sensor, damaged wiring, poor electrical connections, or a problem within the electronic throttle body.

Symptoms

  • Check Engine Light (MIL) illuminated
  • Poor acceleration
  • Reduced engine power (Limp Mode)
  • Rough idle
  • Throttle hesitation
  • Hard starting in some cases
  • Unstable engine speed

Possible Causes

  • Faulty Throttle Position Sensor (TPS)
  • Damaged or corroded connector
  • Open or short circuit in TPS wiring
  • Faulty electronic throttle body
  • 5V reference circuit problem
  • Poor sensor ground
  • ECM failure (rare)

Typical Circuit

ECM (5V Reference)
        │
        ├────────────► TPS Sensor
        │
Ground ─┘
        │
Signal ─────────────► ECM

Signal Flow

Throttle Pedal
       │
       ▼
Throttle Body
       │
TPS Sensor
       │
Voltage Signal
       ▼
ECM
       │
Fuel Injection
Throttle Control
Ignition Timing

How to Diagnose

1. Visual Inspection

  • Inspect the TPS connector for corrosion or loose terminals.
  • Check the wiring harness for damage or broken insulation.
  • Inspect the throttle body for contamination or mechanical sticking.

2. Live Data Test (Autel)

  • Open Diagnosis → Engine → Live Data.
  • Select Throttle Position Sensor (TPS) or Throttle Position (%).
  • With the ignition ON and engine OFF, slowly press the accelerator pedal.
  • The TPS value should increase smoothly from approximately 10–20% at closed throttle to 80–100% at wide open throttle (values vary by manufacturer).
  • If the reading is fixed, erratic, or drops suddenly, inspect the TPS circuit or sensor.
  • If available, compare TPS Sensor A with TPS Sensor B. Both should change smoothly while maintaining the expected relationship.

3. Multimeter Test

  • Verify the 5V reference voltage.
  • Check sensor ground continuity.
  • Measure the TPS signal voltage while slowly opening the throttle.
  • The voltage should increase smoothly without sudden drops or spikes.

4. Smoke Test (If Applicable)

A smoke test is not normally required for P0120. However, if idle instability or lean condition codes are also present, inspect the intake system for vacuum leaks using a smoke machine.

Common Repairs

  • Replace the faulty TPS sensor (if serviceable).
  • Replace the electronic throttle body if the TPS is integrated and defective.
  • Repair damaged wiring or connectors.
  • Clean the throttle body if excessive carbon buildup is present.
  • Clear the DTC and perform a throttle relearn procedure if required by the manufacturer.

Severity

High. P0120 can cause reduced engine power, poor throttle response, and limp mode, making the vehicle difficult or unsafe to drive under certain conditions.

Important Note

Throttle Position Sensor wiring, voltage ranges, connector pinouts, and relearn procedures vary by manufacturer. Always consult the factory service information for the specific vehicle before performing diagnostics or repairs.

Comments

Post a Comment