Important: The temperature stick is a pencil-like device that has a wax material containing certain chemicals which melt at a given temperature. Use the temperature sticks to determine a thermostat's operating temperature by rubbing 87°C (188°F) and 97°C (206°F) sticks on the thermostat housing. The marks made by the sticks should melt when coolant temperatures reach 87°C (188°F) and 97°C (206°F), respectively. These temperatures are the normal operating range of the thermostat. If the coolant flows as indicated in the table, the thermostat may be worn.

1. Remove the radiator cap, if equipped.
2. Rub a 97°C (206°F) J 24731 on the thermostat housing.
3. Warm up the engine at fast idle.
4. Check for coolant flow before the mark begins to melt.

Is there coolant flow before the mark melts?

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Go to Step 2

Go to Step 3

The problem is caused from something other than the thermostat.

Does the engine still overheat?

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Go to Engine Overheating

System OK

1. Replace the thermostat.

•	Refer to Engine Coolant Thermostat Replacement for 8.1L.

•	Refer to Caterpillar® 3126 Unit Repair Manual for 7.2L.

2. Check the system.

Does the engine still overheat?

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System OK