A P0121 – Throttle Position Sensor (TPS) Range/Performance Fault on the 2018 MG ZS indicates that the Engine Control Module (ECM) has detected that the signal from the throttle position sensor is outside the expected operating range or is not plausible when compared with other engine operating parameters.

On the MG ZS electronic throttle control (drive-by-wire) system, the throttle position sensor continuously reports the throttle plate angle to the ECM, allowing it to accurately control fuel injection, ignition timing, idle speed, torque management, and electronic throttle operation.

The ECM continuously compares the TPS signal with the Accelerator Pedal Position (APP) sensor, Manifold Absolute Pressure (MAP) sensor, Mass Air Flow (MAF) sensor, and engine speed to verify signal accuracy. If the TPS voltage is erratic, inconsistent, or does not correlate with these inputs, Diagnostic Trouble Code P0121 is stored and the Check Engine Light is illuminated.

The vehicle may also enter reduced power (limp-home) mode to protect the engine and electronic throttle system. Common causes include a faulty throttle position sensor, carbon buildup inside the throttle body, damaged wiring, poor electrical connections, or an internal electronic throttle body fault.

Symptoms

• Check Engine Light illuminated
• Diagnostic Trouble Code (DTC) P0121 stored
• Reduced engine power (limp mode)
• Hesitation during acceleration
• Poor throttle response
• Rough or unstable idle
• Engine stalling
• Surging or inconsistent engine speed
• Difficulty maintaining vehicle speed
• Additional electronic throttle control or accelerator pedal Diagnostic Trouble Codes may also be present

Possible Causes

The fault may be caused by a defective Throttle Position Sensor (TPS), internal wear within the electronic throttle body, carbon deposits preventing smooth throttle plate movement, damaged wiring harnesses, loose or corroded electrical connectors, poor ground connections, intermittent 5-volt reference voltage faults, damaged Accelerator Pedal Position (APP) sensor circuits, Engine Control Module (ECM) faults, or CAN Bus communication issues affecting electronic throttle control.

Vacuum leaks, throttle body contamination, and signal correlation faults between the TPS, MAP, and MAF sensors may also trigger DTC P0121.

Diagnosis

Diagnosis begins by connecting a professional diagnostic scan tool to retrieve DTC P0121 together with any related Accelerator Pedal Position (APP), Manifold Absolute Pressure (MAP), Mass Air Flow (MAF), throttle actuator, or electronic throttle control Diagnostic Trouble Codes.

Freeze-frame data and live data should be analysed to monitor throttle position percentage, accelerator pedal position, engine load, engine speed, manifold pressure, airflow, fuel trims, and battery voltage.

The technician should verify that the throttle position changes smoothly from idle to wide-open throttle without sudden spikes, dropouts, or implausible values. The ECM also checks whether the TPS signal is rational when compared with MAP and MAF sensor readings.

A P0121 fault is not always caused by a failed throttle position sensor. Technicians should inspect the throttle body for excessive carbon buildup, sticking throttle plates, damaged gears within the electronic throttle actuator, and contamination that may restrict movement.

The TPS and throttle actuator connectors should be inspected for corrosion, moisture intrusion, bent terminals, damaged insulation, or loose connections.

Electrical testing should include verification of the 5-volt reference, ground integrity, signal voltage, continuity testing, voltage drop testing, and oscilloscope analysis where necessary. If applicable, a throttle body cleaning procedure followed by an electronic throttle relearn or adaptation procedure should be performed before component replacement.

If the throttle body assembly and associated electrical circuits operate within specification, further diagnosis should include evaluation of the Engine Control Module, Accelerator Pedal Position sensor circuits, engine management system, and the CAN Bus communication network.

Access to OEM wiring diagrams, connector pinouts, throttle body specifications, live data parameters, electronic throttle adaptation procedures, component locations, and manufacturer diagnostic flowcharts is essential for accurate fault isolation.

Following the OEM troubleshooting workflow helps identify the root cause efficiently while avoiding unnecessary replacement of throttle bodies, sensors, wiring harnesses, accelerator pedal assemblies, or electronic control modules.

Recommended Resources

OEM Service and Repair Manual

The OEM Service and Repair Manual is the most reliable resource for diagnosing and repairing a P0121 Throttle Position Sensor Range/Performance Fault. It typically includes:

• Diagnostic procedures specific to DTC P0121
• Electronic throttle control system operation
• Throttle Position Sensor (TPS) testing procedures
• Electronic throttle body inspection and cleaning procedures
• Accelerator Pedal Position (APP) sensor diagnostics
• 5-volt reference, signal, and ground circuit testing
• Throttle body adaptation and relearn procedures
• Engine management wiring diagrams and circuit descriptions
• Connector pinout information
• Component locations and removal procedures
• OEM troubleshooting workflows
• Repair specifications and electrical testing procedures

To order the correct OEM Service and Repair Manual for your 2018 MG ZS, submit your request using the form below and our team will provide the ordering details.