Systems and methods allow detection and location of die coolant leaks while a die is in a die cast machine and at operating temperature. The testing can be performed during normal down time of the die casting assembly and repeated as desired. Cooling circuits of the die can be tested in zones where zones are sorted to identify particular cooling circuits as having leaks as necessary. Valving, leak sensors, air decay units, and added machine control are provided in the die cooling system to enable onboard leak testing of the die while installed in the die casting machine.