Thermo king c600 coolant out from overflow

Age and hours on the unit? I would first check the fan and make sure that the radiator fins are clean. Pressure test the system. A thermostat would be a good idea. Rare but possible is an issue with the water pump such as the impeller being loose on the shaft. I would want to know the coolant temp when the alarm is triggered. If high, then overheating is causing the coolant loss. Otherwise getting air in the system or another problem such a head gasket leak.

 

Could be a head gasket. C600's are known for having problems with the head gaskets. How old is the unit and how many hours? I could be under warranty.

 

Set it at -20 and park the menu on water temp. Watch how the temp rises. Might have to watch for an hour or so to see if the temp spikes at any time or spikes on the way up. If so it's likely just the thermostat.

 

Did you bleed the system after changing the thermostat? See file in post #2 for procedure.
If it was the water pump which was causing low or no coolant flow it would overheat. But as you say the temp at time of overflow was 160F assuming the temp reading is correct. You can loosen the belts and see if there is play in the water pump by grabbing each side of the fan and try to move back and forth.
When you say "tested with block tester" do you mean the blue fluid that changes color if it contacts combustion/exhaust gases? If so they are not always accurate and will not always show a problem when one actually exists. Better ways are having someone check with an exhaust gas analyzer over the radiator with the cap off. Or an adaptor attached to the radiator to check pulse (any gasses being pumped into the cooling system) with a scope.
At this point I would pressure test cold and at operating temp. Could also check under vacuum with a cooling system vacuum filler. Either drawing in air, head gasket or cracked head or block. Puzzling that the problem is intermittent.

 

Don't even waste any more time testing anything.100 percent head gasket.Those engine are infamous for it.