Ok, assuming the Cat is the same as the Doosan which it more than likely is. It sounds like the problem is in either the logic card or the base drive circuit (which is located on a seperate small pcb board). The base drive consists of 3 transistors T314, T315, T316. T316 gets it's base signal from the logic card, a 1.5V pulse, pin P12-5. This turns on T316 causing the other 2 transistors to turn on. So check the 3 transistors, in particular the base emmiter, and collector emmiter. These transistors are very cheap to replace. Check for a pulse from the logic card.
Regarding the power transistors T1D&2 theses are darlington transistors and must be changed in pairs as they need to matcched otherwise you can get thermal runaway which will destroy one of the power transistors.