A P0122 – Throttle Position Sensor (TPS) Circuit Low Input fault on the 2018 MG ZS indicates that the Engine Control Module (ECM) has detected a throttle position sensor signal voltage that is lower than the manufacturer’s specified operating range.

The MG ZS uses an electronic throttle control (drive-by-wire) system in which the throttle position sensor continuously reports the throttle plate angle to the ECM. This information is used to regulate fuel injection, ignition timing, idle speed, torque management, transmission shift strategy, and emissions control.

When the TPS signal falls below the expected threshold, the ECM interprets the throttle position as implausibly low or detects a circuit fault.

As a result, Diagnostic Trouble Code P0122 is stored, the Check Engine Light is illuminated, and the vehicle may enter reduced power (limp-home) mode to protect the electronic throttle system. Common causes include a short-to-ground in the TPS signal circuit, a faulty throttle position sensor, damaged wiring, poor electrical connections, or an internal electronic throttle body failure.

Symptoms

• Check Engine Light illuminated
• Diagnostic Trouble Code (DTC) P0122 stored
• Reduced engine power (limp mode)
• Poor throttle response
• Hesitation during acceleration
• Rough or unstable idle
• Engine stalling
• Limited engine speed or restricted throttle opening
• 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), an internal failure of the electronic throttle body, a short-to-ground in the TPS signal circuit, damaged wiring harnesses, loose or corroded electrical connectors, poor ground connections, loss of the ECM 5-volt reference supply, damaged Accelerator Pedal Position (APP) sensor circuits, carbon buildup restricting throttle plate movement, Engine Control Module (ECM) faults, or CAN Bus communication issues affecting electronic throttle control. Electrical shorts and throttle body faults are among the most common causes of DTC P0122.

Diagnosis

Diagnosis begins by connecting a professional diagnostic scan tool to retrieve DTC P0122 together with any related Accelerator Pedal Position (APP), throttle actuator, Manifold Absolute Pressure (MAP), Mass Air Flow (MAF), 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, battery voltage, fuel trims, and throttle actuator command.

The technician should verify that the throttle position sensor voltage changes smoothly throughout throttle movement. A TPS signal that remains significantly below specification or drops below the expected minimum voltage indicates a low input fault requiring further electrical diagnosis.

A P0122 fault is not always caused by a failed throttle position sensor. Technicians should inspect the throttle body assembly for carbon deposits, sticking throttle plates, damaged internal gears, or contamination that may interfere with throttle movement.

The TPS connector and wiring harness should be examined for corrosion, moisture intrusion, damaged insulation, bent terminals, or loose connections. Electrical testing should include verification of the 5-volt reference circuit, sensor ground, signal voltage, continuity testing, voltage drop testing, and oscilloscope analysis where necessary.

If excessive carbon buildup is present, the throttle body should be cleaned and an electronic throttle adaptation or relearn procedure performed before replacing components.

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 P0122 Throttle Position Sensor Circuit Low Input fault. It typically includes:

• Diagnostic procedures specific to DTC P0122
• 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.