The ECM detects the following conditions:

•The engine coolant temperature is greater than 106 °C (223 °F) for the C-10 or C-12.
•The engine coolant temperature is greater than 111 °C (232 °F) for the 3406E.
•The engine has been running for at least 30 seconds.

Probable Causes:

1) Low coolant level
2) Improper antifreeze (Wrong mix/type)
3) Air in the cooling system
4) Fan clutch failure/malfunction
5) Coolant temperature sensor/harness
6) Radiator failure
7) Radiator filler cap failure
8) Charge air cooler failure
9) Fan, fan belt, or fan shroud failure
10) Restriction in air inlet or exhaust system
11) Water temperature regulator
12) Water pump failure
13) High outside temperature or high altitude
14) Engine being run in “lug condition”