Diagnostic Overview
Cummins fault code 118 is commonly related to coolant temperature signal issues, depending on engine family and calibration. Although listed here under fuel, incorrect coolant temperature data can affect fueling, fan control, cold-start strategy, derate logic, and aftertreatment operation. Causes include a failed coolant temperature sensor, low coolant level, thermostat problems, overheating, damaged wiring, or poor connector contact. Diagnosis should include comparing live coolant temperature to actual engine temperature, checking coolant level, and inspecting the sensor circuit.