A P0118 – Engine Coolant Temperature (ECT) Sensor Circuit High Input fault on the 2018 MG ZS indicates that the Engine Control Module (ECM) has detected a voltage signal from the Engine Coolant Temperature sensor that is higher than the manufacturer’s calibrated operating range.

The ECT sensor is a negative temperature coefficient (NTC) thermistor that monitors engine coolant temperature and provides the ECM with critical information for fuel injection, ignition timing, cold-start enrichment, idle speed control, radiator cooling fan operation, and emissions management.

Because the ECT sensor’s resistance decreases as coolant temperature rises, a high input voltage is interpreted by the ECM as an implausibly low coolant temperature, often around -40°C (-40°F). When this condition is detected, the ECM stores Diagnostic Trouble Code P0118 and illuminates the Check Engine Light. Common causes include an open circuit, disconnected sensor, damaged wiring, poor electrical connections, a failed ECT sensor, or an internal ECM fault.

Symptoms

• Check Engine Light illuminated
• Diagnostic Trouble Code (DTC) P0118 stored
• Hard starting, especially when the engine is warm
• Rough or unstable idle
• Increased fuel consumption
• Black exhaust smoke caused by an excessively rich air-fuel mixture
• Reduced engine performance
• Poor throttle response
• Radiator cooling fan operating continuously
• Additional cooling system or fuel system Diagnostic Trouble Codes may also be present

Possible Causes

The fault may be caused by a defective Engine Coolant Temperature (ECT) sensor, an open circuit in the ECT signal or ground wire, disconnected sensor connector, loose or corroded electrical terminals, damaged wiring harnesses, excessive circuit resistance, poor connector contact, Engine Control Module (ECM) faults, or CAN Bus communication issues affecting engine management. Low engine coolant level, thermostat faults, or previous cooling system repairs that have damaged the sensor wiring may also contribute to the condition. Open circuits and failed ECT sensors are among the most common causes of DTC P0118.

Diagnosis

Diagnosis begins by connecting a professional diagnostic scan tool to retrieve DTC P0118 together with any related thermostat, cooling fan, fuel trim, ignition, or misfire Diagnostic Trouble Codes. Freeze-frame data and live data should be analysed to monitor engine coolant temperature, intake air temperature, engine speed, fuel trims, battery voltage, radiator fan command, and thermostat operation.

If the Engine Coolant Temperature reading immediately displays an implausibly low value (typically around –40°C) with the ignition switched on, the technician should suspect an open circuit, disconnected sensor, or failed ECT sensor before replacing cooling system components.

A P0118 fault is not always caused by a failed Engine Coolant Temperature sensor. Technicians typically inspect the coolant level, thermostat operation, radiator, cooling fan, wiring harness, and sensor connector for loose terminals, corrosion, damaged insulation, broken wires, or connector damage before replacing the sensor.

The ECT sensor should be tested for correct resistance relative to coolant temperature while verifying the ECM 5-volt reference, signal circuit, ground circuit, battery voltage, and connector integrity. Electrical continuity testing, resistance testing, and connector pin inspection are commonly performed to identify intermittent open circuits or excessive resistance affecting the ECT signal. The cooling system should also be inspected for coolant leaks or trapped air that may influence sensor operation.

If the Engine Coolant Temperature sensor and associated electrical circuits operate within specification, further diagnosis should include evaluation of the Engine Control Module, thermostat control system, cooling fan circuits, engine mechanical condition, and the CAN Bus communication network.

Access to OEM wiring diagrams, connector pinouts, ECT circuit descriptions, live data specifications, cooling system diagrams, sensor resistance charts, 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 sensors, wiring harnesses, cooling system components, 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 P0118 Engine Coolant Temperature Sensor Circuit High Input fault. It typically includes:

• Diagnostic procedures specific to DTC P0118
• Engine management wiring diagrams and circuit descriptions
• Engine Coolant Temperature (ECT) sensor testing procedures
• ECT sensor resistance and voltage specifications
• 5-volt reference, signal, and ground circuit diagnostics
• Cooling system inspection and thermostat testing procedures
• Radiator cooling fan control diagnostics
• Connector pinout information
• Component locations and removal procedures
• OEM troubleshooting workflows
• Repair specifications and electrical testing procedures

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