4040 heater blows cold

Thermostats, either their stuck open or missing, quicker fix put cardboard in front of the radiator, just keep an eye on the coolant temp gauge. Make sure it doesn't overheat
 
This will sound stupid but check the water level. If the cooling system is low the cab heater is the first thing to not work.

If it is full then is the engine coming up to operating temperature. IF not replace/install new thermostats.

Now if it is coming up to operating temperature check and see if the supply hose going up to the cab is warm/hot. The supply hose will come off the top of the motor and the return will be lower on the block. If it is warm then your going to have to raise the roof to check out the temperature control valve. When you turn the temperature control knob it is moving a cable that goes to the control valve. Look this over and see if it is working. Cable break, valves stick, the arm on the valve comes lose from the valve itself, and etc.

I have not seen one with a working valve that would not heat when full and at operating temperature. Even with the AC turned on high the heater will blow warm air.
 
One should be hot, one a little cooler if operating correctly.

Confirm engine is hot.
If both hot, then problem is airflow not going through heater core.
If one hot and other cold, or if both cold then no coolant flowing through core.

Super simple.
 

Check heater hoses for manual cutoff valves. My 4255 has had the "aftermarket heater hose manual cutoff valves" closed for yrs as normally a heater on cab tractor isn't required much in Texas winters
 

Hot supply to heater should be connected as close to the thermostat BUT on the engine side of the stat, not the radiator side of the stat. Highest pressure and hottest .
Return line from heater should go to a threaded port on the inlet side of the water pump. Lowest pressure point .
 

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