OBD2 Code P0118: Engine Coolant Temperature Sensor Circuit High Input – Diagnosis and Repair Guide

What Does P0118 Really Mean?

The OBD2 trouble code P0118, known as “Engine Coolant Temperature Sensor Circuit High Input,” indicates a problem within your vehicle’s Engine Coolant Temperature (ECT) sensor circuit. This sensor is crucial for monitoring the engine’s temperature and relaying this information to the Powertrain Control Module (PCM), the car’s computer.

The ECT sensor, typically a thermistor, is located in the engine coolant passage. Its resistance changes with temperature: high resistance when the coolant is cold and low resistance when it’s hot. The PCM supplies a 5-volt reference voltage to the sensor and monitors the voltage drop to determine the coolant temperature.

A P0118 code is triggered when the PCM detects an abnormally high voltage signal from the ECT sensor circuit. This “high input” reading suggests that the sensor is reporting an excessively low coolant temperature, even when the engine should be warm. This can happen if the sensor circuit is open, shorted to voltage, or if the sensor itself is faulty. Effectively, the PCM perceives a signal indicating the coolant is much colder than it actually is, or should be under normal operating conditions.

Understanding the Symptoms of a P0118 Code

When your vehicle throws a P0118 code, you might observe several symptoms affecting engine performance and fuel efficiency. These symptoms can range from minor inconveniences to significant drivability issues:

• Poor Fuel Economy: The PCM relies on accurate temperature readings to optimize fuel delivery. A false cold reading from the ECT sensor can cause the PCM to enrich the fuel mixture excessively, leading to wasted fuel and decreased mileage.
• Engine No-Start Condition: In some cases, a P0118 code can prevent the engine from starting altogether. The PCM might interpret the extremely low temperature reading as conditions unsuitable for starting, inhibiting ignition or fuel delivery.
• Rough Engine Running: If the engine does start, it might run very poorly. Symptoms can include black smoke from the exhaust (due to a rich fuel mixture), rough idling, misfires, and overall unstable engine performance. The incorrect fuel-air mixture and timing adjustments based on faulty temperature data are the primary culprits.
• Check Engine Light (MIL) Illumination: The most direct symptom is the illumination of the Malfunction Indicator Lamp (MIL), commonly known as the “Check Engine Light,” on your dashboard. This light signals that the PCM has detected a problem and stored a diagnostic trouble code, in this case, P0118.

Pinpointing the Causes of OBD2 Code P0118

Several potential issues can lead to a P0118 code. Systematic diagnosis is needed to accurately identify the root cause. Here are the common culprits:

• Faulty ECT Sensor: The sensor itself might be defective. Internal shorts or open circuits within the sensor can cause it to send incorrect readings to the PCM. This is often the most frequent cause of P0118.
• Wiring and Connector Issues: Problems in the wiring harness or connector leading to the ECT sensor are common. These can include:
  • Bad Connection at the Sensor Connector: Corrosion, loose pins, or damage to the connector can disrupt the electrical signal.
  • Open Circuit in the Ground Wire: A break or open in the ground wire between the ECT sensor and the PCM can cause a high resistance and incorrect voltage readings.
  • Short Circuit in the Voltage Feed Wire: The voltage feed wire might be shorted to a voltage source, leading to an abnormally high voltage signal reaching the PCM.
• PCM Malfunction (Less Likely): While less frequent, a faulty PCM can misinterpret sensor signals or have internal issues affecting the ECT sensor circuit. This is usually considered after ruling out other potential causes.

Step-by-Step Solutions for Resolving a P0118 Code

Diagnosing and fixing a P0118 code requires a methodical approach. Follow these steps to troubleshoot and resolve the issue:

1. Initial Scan Tool Check: Use an OBD2 scan tool to confirm the presence of the P0118 code and check the live data reading from the ECT sensor. Does the temperature reading seem plausible? If the reading is erratic or illogical, it points towards a sensor or circuit problem. If the reading appears normal initially, the fault might be intermittent.

2. “Wiggle Test” for Intermittent Issues: If the scan tool reading seems plausible but you suspect an intermittent problem, perform a “wiggle test.” With the scan tool still connected and monitoring the ECT sensor reading, gently wiggle the sensor connector and wiring harness. Observe the scan tool data for any sudden drop-outs or changes in the temperature reading. Fluctuations indicate a loose connection or wiring fault.

3. Sensor Resistance Check: If the scan tool shows an illogical temperature reading, disconnect the ECT sensor and use a multimeter to measure its resistance. Compare the measured resistance to the manufacturer’s specifications for the sensor at the current ambient temperature (or ideally at a known coolant temperature if the engine is warm – be careful!). If the resistance is significantly out of specification, replace the ECT sensor.

4. Circuit Continuity and Voltage Checks: If the sensor resistance is within spec, the problem likely lies in the wiring circuit.

   • Check for 5V Reference Voltage: With the sensor disconnected and ignition ON, use a multimeter to check for the presence of the 5-volt reference voltage at the sensor connector. There should be 5V on one of the terminals (refer to your vehicle’s wiring diagram).
   • Check for Ground Continuity: Verify ground continuity at the other terminal of the sensor connector using the multimeter.
   • Jumper Test (If No Voltage/Ground): If you are missing the 5V reference or ground, use a fused jumper wire to briefly jumper the two terminals of the sensor connector together with the sensor disconnected. Now check the scan tool reading. The temperature reading should now max out to a very high value (e.g., above 250°F or 120°C). If it does not max out, there is likely a problem with the ground circuit or the 5V supply circuit between the PCM and the sensor.

5. PCM Connector Checks (If Circuit Issue Found): If you suspect a wiring issue between the PCM and the sensor, locate the PCM connector and check for 5V reference and ground continuity at the PCM connector pins that correspond to the ECT sensor circuit (again, refer to wiring diagrams).

   • Wiring Repair: If 5V and ground are present at the PCM connector but not at the sensor connector, there is an open or short circuit in the wiring between the PCM and the sensor. Repair the wiring as needed.
   • PCM Pin Voltage Check: If 5V reference is not present at the PCM connector pin, carefully remove the wire from the PCM pin and check for voltage directly at the PCM pin itself. If voltage is now present at the pin after removing the wire, there might be a short to ground in the wiring harness. If voltage is still absent at the PCM pin, a faulty PCM might be the cause (though this is less common).

Important Note: Incorrect PCM diagnosis can lead to unnecessary replacements. If you are not comfortable performing PCM diagnostics, it’s best to consult a qualified automotive technician. Often, a P0118 code is resolved by simply replacing a faulty ECT sensor or repairing a wiring issue.

Related OBD-II Codes:

• P0115 – Engine Coolant Temperature Circuit Malfunction
• P0116 – Engine Coolant Temperature Circuit Range/Performance Problem
• P0117 – Engine Coolant Temperature Circuit Low Input
• P0119 – Engine Coolant Temperature Circuit Intermittent
• P0125 – Insufficient Coolant Temperature For Closed Loop Fuel Control
• P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Disclaimer: This information is for informational purposes only and should not be considered professional repair advice. We are not liable for any actions taken on your vehicle based on this information. Always consult a qualified technician for diagnosis and repair. All content is copyright protected.

For Further Assistance:

Do you still have questions about the P0118 code? Join our car repair forums for free and post your question to get help from the community!