P0118 on the 6.7L Power Stroke is an engine coolant temperature (ECT) sensor code. It signals an out-of-range high voltage on the ECT circuit, which the ECM interprets as an implausible temperature reading. Since coolant temperature drives fueling, cooling, and warnings, it is important to get right.
What P0118 means
P0118 is defined as Engine Coolant Temperature Sensor Circuit High. The ECT sensor is a thermistor whose resistance drops as coolant warms. A high circuit voltage corresponds to an open circuit or an extremely cold reading. When the ECM detects this high, out-of-range signal, it sets P0118. This is an electrical code, so it usually reflects the sensor or its wiring rather than an actual cooling problem.
Common causes
- Faulty ECT sensor
- Open or high-resistance wiring in the ECT circuit
- Corroded or disconnected ECT connector
- Broken sensor terminal or backed-out connector pin
- Chafed signal wire that has opened
Symptoms
- Check engine light with stored P0118
- Temperature gauge reading low or erratic
- Hard cold starts or extended warm-up enrichment
- Cooling fan behavior that seems off
- Reduced fuel economy from incorrect temperature data
How to diagnose it
Read live ECT data; P0118 usually shows an implausibly cold reading even after the engine warms up. Inspect the connector for corrosion, loose pins, or a poor fit, and check the wiring for opens and high resistance. Measure the sensor's resistance against a temperature-versus-resistance chart. If the wiring and ground are good but the sensor reads open, the sensor is faulty. Confirm actual coolant temperature with an infrared thermometer to be sure the reading is wrong.
How to fix it
Replace the ECT sensor if it tests open or out of range. Repair open or high-resistance wiring and clean or replace a corroded connector, seating the pins properly. After the repair, clear the code and confirm the ECT reading rises normally with engine temperature and matches actual coolant temperature. Verify the code does not return after a warm-up and drive cycle.
Frequently asked questions
Does P0118 mean my engine is overheating?
No. P0118 is a high-voltage circuit fault that usually reads as too cold, not too hot. It typically means a sensor or wiring problem rather than an actual overheat.
Can P0118 affect cold starts?
Yes. If the ECM thinks the coolant is very cold, it may alter fueling and warm-up behavior, which can make starting and drivability inconsistent.
Is the ECT sensor easy to replace?
On most 6.7L Power Stroke trucks it is a straightforward sensor swap once located, though you should catch coolant and refill as needed.
Need parts or a second opinion? The team at Core Diesel Parts can help you match the right components to your engine. Email sales@coredieselparts.com with your VIN and symptoms and we will point you to the correct parts.
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