P0118 on the LML Duramax indicates the engine coolant temperature (ECT) sensor circuit is reading a high input, which the ECM interprets as an implausibly cold reading or an electrical fault. The ECT signal affects fueling, cooling fan control, and emissions strategies, so a fault here can influence how the engine runs and warms up. Here is how to diagnose and fix P0118.
What P0118 means
P0118 is "Engine Coolant Temperature Sensor 1 Circuit High Input." The ECT sensor is a thermistor whose resistance falls as coolant warms. A high-input reading corresponds to very high signal voltage, which typically indicates an open circuit, a failed sensor, or a wiring fault rather than a real temperature. The ECM relies on this reading to manage warm-up fueling, the cooling fan, and emissions functions.
Common causes
- Open circuit in the ECT signal or ground wiring
- A failed ECT sensor
- Corroded or disconnected sensor connector
- Chafed or broken wiring near the sensor
- Poor ground connection
- Rarely, an ECM input fault
Symptoms
- Check engine light with P0118 stored
- Hard cold starting or extended warm-up
- Cooling fan running more than expected
- Rough running while cold
- Poor fuel economy
- Inaccurate temperature gauge behavior
How to diagnose it
Scan all codes and view the ECT reading in live data; a reading stuck at an implausibly cold value regardless of engine temperature suggests an open circuit or failed sensor. Inspect the sensor connector and wiring for opens, corrosion, and chafing. With the connector jumped per the factory procedure, the reading should swing to hot, helping confirm whether the sensor or wiring is at fault. Measure the sensor's resistance at a known temperature and compare to specification. Verify the ground circuit is intact.
How to fix it
Repair any open or chafed wiring and clean or replace a corroded connector. If the sensor reads out of specification, replace the ECT sensor. Because a false-cold reading changes fueling and fan operation, restoring an accurate signal often improves cold running and economy. After the repair, clear the code and confirm the ECT reading matches actual coolant temperature and rises normally as the engine warms.
FAQ
What does a high input mean on the ECT?
It usually means the circuit is reading very high voltage, typically from an open circuit or failed sensor, which the ECM sees as an implausibly cold temperature.
Can P0118 make the truck run poorly cold?
Yes. A false-cold reading can enrich fueling and keep the engine from warming up efficiently, affecting running quality and economy.
Is the ECT sensor easy to replace?
It is a straightforward, low-cost component. Confirm the correct part for your LML and take care with the cooling system when replacing it.
Need parts to fix this code? The team at sales@coredieselparts.com can match the correct sensor, injector, or emissions component to your engine and model year.
Need the part?
We stock the parts these trucks need. Shop all LML Duramax parts →