Fix P0118: Mechanic’s solutions that WORKED

The code P0118: Engine Coolant Temperature (ECT) Sensor Circuit High Input, indicates a fault in the ECT sensor circuit, suggesting that the Engine Control Module (ECM) is receiving a high input voltage from the sensor. This is most likely caused by a malfunctioning ECT sensor, damaged wiring, or a bad connection.

P0118 quick overview

Meaning	P0118: Engine Coolant Temperature (ECT) Sensor Circuit High Input
Is it serious?	Yes, it can be. If the ECM cannot accurately determine the engine’s coolant temperature, it might result in poor engine performance, increased emissions, and potential damage to the engine in the long run.
Possible causes	– Faulty Engine Coolant Temperature sensor. – Wiring or circuit issue leading to the ECT sensor (like a short to power). – Poor electrical connection in the ECT sensor circuit. – An open in the ECT ground circuit or high resistance. – Faulty Engine Control Module (though this is rare).
How to diagnose?	Check for Other Codes: Before diagnosing P0118, see if there are other codes present that might provide more information. Visual Inspection: Check the ECT sensor and surrounding area for any signs of damage, corrosion, or loose connections. Test the ECT Sensor: Using a multimeter, test the ECT sensor’s resistance and compare it to the manufacturer’s specifications. Check Wiring and Connectors: Inspect the wiring leading to the ECT sensor for any signs of damage, burns, or shorts. Ensure connectors are clean and secure. Scan Tool Diagnosis: Use a scan tool to observe the live data from the ECT sensor. If the reading is consistently high, even when the engine is cold, the sensor might be faulty. Check the Coolant: Ensure the engine’s coolant levels are correct and there are no air pockets in the cooling system, as these can give incorrect readings. Replace and Retest: If the ECT sensor is found to be faulty, replace it. Clear the codes and test drive the vehicle to see if the code reappears. If the problem persists, consider further diagnosis or seek professional assistance.

How to fix P0118 – learn from mechanics

I looked into P0118 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Saturn

Vehicle Information:
1997 Saturn SL2 1.9L

Issue + Repair History:

• Past issue with false code P0711 (A/T temp) due to flat towing behind a motor home; a TSB was found relating to this issue.
• Current problem: The vehicle now shows the ECT code P0118 (value high) and the P0711 code.
• During 1-2 hour highway trips, the temperature gauge reads hot, and the MIL light comes on.
• Previous repair: The temperature sending unit was replaced by another shop in 9/2002.

Suggestions from the Community:

• Check the temperature sensor to see if it’s cracked or has leakage causing shorting in the connector.
• There might be an overheating issue on long drives if the radiator is plugged.
• Inspect the coolant temp sensor for cracks.

Final Fix:
The temperature sending unit and its connector were found to be the culprits. The unit had recently been replaced, but the connector was damaged. After fixing these issues, the problem was resolved.

Case #2: Volkswagen

Vehicle Information:
2000 Volkswagen Jetta GLX
Engine: 2.8L, SOHC
Transmission: 5-speed Standard Transaxle

Issue + Repair History:

• Initial problem: Check engine light on with code P1296.
• Repair history: Code cleared, returned to the customer, but the car came back 2 days later with code P0118. The code was cleared, the car was test-driven, and code P1296 reappeared.

Suggestions from the Community:

• Most technicians suggested that both codes (P1296 and P0118) relate to the engine coolant temperature malfunction.
• The majority recommended replacing the coolant temperature sensor.
• Some technicians suggested conducting a wiggle test on the sensor connector to check for loose connections.
• A few technicians advised checking the thermostat, indicating it might be opening prematurely.

Final Fix:
The coolant temperature sensor and the thermostat were replaced. Wiring was checked and found to be okay. After these repairs, the problematic code did not return.

Case #3: Chrysler

Vehicle Information:
1999 Chrysler Town & Country LX
Engine: 3.8L
Main Issue: MIL Lamp on

Issue + Repair History:

• MIL lamp on with codes P0118 and P1478.
• Discrepancies in coolant temperature readings and voltages between different test conditions (KOEO vs. KOER).
• Scanner indicated fluctuating battery temperatures.
• No prior related repair history was mentioned.

Suggestions from the Community:

• Mechanic 1: Ensure IAT sensor and CTS plugs aren’t switched; inspect evaporator connections under the dash.
• Mechanic 2: Inspect the intake air temperature sensor due to a common ground with the PCM.
• Mechanic 3: Examine the correlation between battery temperature, engine coolant temperature, and intake air temperature sensor.
• Mechanic 4: Check for electrolysis in the coolant system; inspect A/C voltage between the radiator and battery ground.
• Mechanic 5: Suggested tests involving the disconnection and potential replacement of the ECT and thermostat.
• Mechanic 6: Questioned the method of ground testing and suggested using a voltage drop test.
• Mechanic 7: Recommended probing the PCM pin for the coolant temp sensor to ensure accurate readings.

Final Fix:
The root cause was found to be a creature chewing through the downstream O2 sensor harness, causing exposed wires to touch. The solution involved repairing the harness and replacing the O2 sensor.

Case #4: Hyundai

Vehicle Information:
2004 Hyundai Santa Fe GLS
Engine: 2.7L
Main Issue: Hesitation, Surge

Issue + Repair History:

• The vehicle experienced a high idle once, which was reset after shutting it off.
• Codes were cleared, and the vehicle was driven for 20 miles before the light came back on.
• There was a hiccup during the drive, but the vehicle was currently running fine.
• Previously, the IAC was replaced for idle issues.
• Past Repairs: T-Belt, Tune-Up.

Suggestions from the Community:

• Mechanic 1: Focus on the ECT (engine coolant temperature) code first.
• Mechanic 2: Inquired about which codes returned after clearing them.
• Mechanic 3: Suggested replacing the throttle position and accelerator position sensors, especially if the code related to them keeps returning.
• Mechanic 4: Check sensors using the 5 volts reference and measure.
• Mechanic 5: Examine the 5-volt reference because many sensors seem to be acting up. Determine common factors among them.
• Mechanic 6: Recommended checking the PID readings to help narrow down the issue.

Final Fix:
The ECT sensor was shorting out with fluctuating readings. After addressing this issue, the vehicle was fixed.

Case #5: Nissan

Vehicle Information:
2006 Nissan Pathfinder SE Off-Road
Engine: 4.0L
Main Issue: Excessive Emissions, No Start

Issue + Repair History:

• The vehicle came from another shop where the engine was replaced.
• Constant coolant code, always in backup mode.
• Tested voltage and found it at 5 volts with a good ground.
• Voltage at the PCM was 2.5v.
• Various test results with changing voltage and temperature feedback.
• The computer was swapped, but the problem persisted.

Suggestions from the Community:

• Mechanic 1: Ensure all PCM grounds are connected and secure after engine replacement. Also, consider replacing the CTS with an OEM part.
• Mechanic 2: Consider the possibility of the system working backward. Unplug various sensors to find the potential culprit, starting with the MAF.
• Mechanic 3: The P0118 code indicates a problem with the Engine Coolant Temperature Sensor Circuit. The issue may be with the coolant temperature sensor itself or the related wiring. Ensure the cooling system is devoid of air pockets.

Final Fix:
The wire passing over the fuel tank and exhaust system in the cross member at the rear of the truck was damaged by the heat of the exhaust system. After fixing this, the issue was resolved.