I have a 1992 Cal Spa PS-4-C hot tub that will not heat up. The indicator light comes on, the contact is coming in, the circulation pump is turning on and there is good flow through the jets.

Probably a bad heating element. Is the element getting voltage? What's the ohm reading through the element?

A Meter and an Amprobe will make short work of this. If You have voltage at Element, and No Current, It is Open. Sometimes the Element will deform and touch Housing and trip GFI. Measuring resistance with no power and one lead off should confirm it is open, Just more work/Time? Good Luck Really shouldn't need to disconnect a lead, it is just good Practice as Contacts can be fused.