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Bringing this thread back from the dead. Would like to attempt to get this thing working. Can anyone help? Thanks!

It has a rocker switch on the outside of the panel. I can switch it to all three positions and still don't get any power. I tested the switch and it checks good. So it appears the problem is elsewhere. Could it be the temp probe? I don't even know where it is at this point. But hopefully it isn't impossible to reach. Thanks!

Found the problem, but don't know the cause of it yet. In the photo you can see both the two pole contactor and the pole and a half contactor. Both are new. The red wire connected to the top of the two pole is not getting any power. I took the cover off the contactor, pushed it in manually, and I get heat and the light. Checked with the meter at the actual heater and full voltage is there. The contactor on the right is working and the yellow wire is hot. I put the yellow wire on the two pole contactor to test it and the contacts pulled in like they are supposed to. So I know the contactor is good. The culprit is the red wire with no power. I've traced it back as far as I can through the pressure switch all the way to the cover panel with the timer and I have continuity throughout. This is as far as I got. I can't determine where the power loss to the red wire is. Any ideas? Thanks again!

Finally received the pressure switch and installed it. No change. Unfortunately the weather has been less than ideal so I wasn't able to check anything else. I'll try to get photos of the pack asap.

Update: Upon further inspection, I discovered the pressure switch is stuck open. The plunger which is activated by the water pressure is stuck at the bottom and doesn't come up. But even when I manipulate the switch manually I don't show connection on the meter. So a replacement switch is ordered and hopefully on the way. We'll see if this solves the problem. I have a feeling there could be more issues. I'll update as soon as I get it installed. Thank you all for the help!

I will get photos as soon as the weather allows. As for the contactors... Before the tub stopped heating, the contactors started clacking non stop. So in hopes to prevent further damage I cut the breaker to the tub and shut it off. This unit has two contactors. One is a pole and a half. The other is a two pole. When I began troubleshooting, I determined the two pole wasn't making connection on one side. When I removed it, I took the cover off and the contactor fell apart. One side was completely broken and in pieces. I thought I had discovered the problem. However, upon replacing the contactor I still have no heat or light. Note: the control panel on this unit has a "ready" and "heat" light, both of which are dark at all times. The "jet" and "blower" lights both are still on and both those features function correctly. I do not see a flow switch like the one in the photo above anywhere on the unit. I do have a multimeter. Thank you all for the comments and feedback!

I have an old Spa Crest spa with a Spa Builders AP1 control and heating unit. Everything works except the heat. The heating element tests good with the multimeter. I am not getting any current to the lugs on the element, however. The air switch also tests good with the meter. Both contactors inside the control unit are new. The reset switch for overheating tests good. Not sure what to test next. This is a mid nineties tub in otherwise good condition. Any help is greatly appreciated!