Common Symptoms
- Check Engine Light illuminated
- Engine runs rich or lean intermittently
- Poor fuel economy
- Hard starting when engine is cold or warm
- Cooling fans run constantly or not at all
- Erratic temperature gauge readings
Most Likely Root Causes
- Damaged or corroded wiring harness to coolant temperature sensor
- Intermittent connection at coolant temperature sensor connector
- Failing engine coolant temperature sensor
- Corroded or loose connector pins at ECM
- Coolant temperature sensor circuit chafing against engine components
Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.
AI walks you through every test — one step at a time. Plans start at $12.95/mo.
Component Location
Engine coolant temperature sensor is typically located near the thermostat housing or on the cylinder head coolant passage.
Manufacturer Notes
Intermittent codes often indicate connector or wiring issues rather than sensor failure and require careful harness inspection.
Also diagnosing J1939 SPN/FMI codes?
Heavy-duty trucks often throw both OBD-II and J1939 faults simultaneously. AI Diesel Tech handles both protocols in a single guided session.
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician. Professional-grade OBD-II and J1939 fault analysis — trusted by technicians nationwide, used in shops, fleets, and training programs.
Get AI-Guided Diagnosis for P0119
The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck.