A P0123 – Throttle Position Sensor (TPS) Circuit High Input fault on the 2018 MG ZS indicates that the Engine Control Module (ECM) has detected a throttle position sensor signal voltage that is higher than the manufacturer’s specified operating range.

The MG ZS utilizes 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 essential for controlling fuel injection, ignition timing, idle speed, torque management, transmission shift strategy, emissions systems, and electronic throttle operation.

Under normal operating conditions, the TPS signal rises smoothly from approximately 0.5 volts at closed throttle to about 4.5 volts at wide-open throttle.

When the ECM detects a signal voltage exceeding the calibrated threshold—typically above 4.8 volts—it stores Diagnostic Trouble Code P0123, illuminates the Check Engine Light, and may place the vehicle into reduced power (limp-home) mode to protect the electronic throttle system. Common causes include a short to the 5-volt reference, an open sensor ground, a faulty throttle position sensor, damaged wiring, or an internal electronic throttle body fault.

Symptoms

• Check Engine Light illuminated
• Diagnostic Trouble Code (DTC) P0123 stored
• Reduced engine power (limp mode)
• Poor throttle response
• Hesitation during acceleration
• Rough or unstable idle
• Engine stalling
• High or erratic idle 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), an internal failure of the electronic throttle body, a short between the TPS signal wire and the 5-volt reference circuit, an open sensor ground circuit, damaged wiring harnesses, loose or corroded electrical connectors, poor terminal contact, 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. Wiring faults that force the TPS signal close to the 5-volt reference are among the most common causes of DTC P0123.

Diagnosis

Diagnosis begins by connecting a professional diagnostic scan tool to retrieve DTC P0123 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 TPS signal changes smoothly throughout throttle movement and does not remain fixed near the maximum voltage.

A signal that stays above approximately 4.8 volts regardless of throttle position strongly indicates a high input fault requiring detailed circuit diagnosis.

A P0123 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 actuator gears, and contamination that may interfere with throttle movement.

The TPS connector and wiring harness should be examined for corrosion, moisture intrusion, damaged insulation, bent terminals, loose connections, or evidence of a short between the signal and reference circuits. Electrical testing should include verification of the 5-volt reference supply, sensor ground integrity, 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 P0123 Throttle Position Sensor Circuit High Input fault. It typically includes:

• Diagnostic procedures specific to DTC P0123
• 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.