LINK A 0319191 - Caterpillar

Pressurized System: Hot coolant can cause serious burns. To open the cooling system filler cap, stop the engine and wait until the cooling system components are cool. Loosen the cooling system pressure cap slowly in order to relieve the pressure.
Stop the engine.
After allowing the engine to cool, check the coolant level. Refer to the Operation and Maintenance Manual.Expected Result:The coolant level is low.Results:
OK - The coolant level is low.Repair: Check the cooling system for leaks. Repair any problems that are found. Refill the cooling system to the proper level.STOP
Not OK - The coolant level is not low.Repair: Air may be trapped in the cooling system. For information on checking the cooling system, refer to the engine's Systems Operation/Testing and Adjusting manual.Ensure that any repairs eliminate the problem.STOP
Not OK - The coolant level is not low.Repair: There may be a problem with the coolant level sensor. Refer to the diagnostic functional test Troubleshooting, "Coolant Level Sensor Circuit - Test".Ensure that any repairs eliminate the problem.STOP