I have a suntek spa Ultra model 501, circa 1991. The heat went out and tested it today. When the jets are on the heater works, but when the jets are off and the pump is in normal filtration mode the heater doesn't work.
In the filtration mode the heater doesn't get power. Can anyone recommend a fix, or is it the control panel and do I have to replace the entire panel?

Do you by chance do anything with the motor wire connections, specifically, perhaps High and Low have been reversed. Most spas will heat in low but not on high. Other possibility is a bad pressure switch, or out of calibration pressure switch. Also try it with out a filter, could be the filter is slowing the flow enough that the pressure switch won't trip in low speed.

Thank you everyone for your help.
I knew it would be the pressure switch from other messages, I tried the filter etc. The bigger problem was that I couldn't find the pressure switch, it was not in any piping. I remove the heater for the hell of it. I noticed three leads coming out of the heater casing instead of 2 that I saw in diagrams. When I opened the heater housing I instantly saw the switch. $30 later and it was good as new. This spa was built by a local manufacturer over 14 years ago and they custom built their heaters, so now I know.