Final answer:
The first thing to check if the LPAC is short cycling is the pressure switch.
Step-by-step explanation:
When an LPAC (Liquid Pump and Air Compressor) is short cycling, the first thing to check is the pressure switch. The pressure switch is responsible for regulating the pressure in the system and turning the LPAC on and off. If the pressure switch is faulty or set incorrectly, it can cause the LPAC to short cycle.