I have a nearly 11 year old Max-E-Therm SR400NA pool heater and last night I turned it on for the first time since last spring (it's now spa time here in Palm Springs). It fired up just fine at first, but the temp display was crazy, jumping all around from 55 degrees to maybe 99 degrees with a new reading about every half second. I had the target temp set to 103 degrees, so this nonsensical series of false reading didn't seem to adversely affect the output of the heater. But when the temp in the spa barely got into the lukewarm range, the heater shut down and displayed an error light. Unfortunately, I wasn't thinking and didn't note whether it was the "service heater" or the "service system" light. Turning off the electricity to the heater for a minute or two and then restarting got me a few more minutes of heat along with the stream of different temps on the display, but eventually it would shut down again with an error light.

Any ideas of why this might be happening? The display is a replacement Insync Engineering membrane that I put in back in 2015 and hasn't given me any problems since. I assume that the display is simply a display, and not the source of the weird readings. A sensor or the computer itself seem more likely. But consider that I replaced the thermal regulator in 2010 because of somewhat similar cycling (defective thermostats fail to add cold water to heated water and then trigger shut high limit downs), and in 2013 with similar problems I replaced it again. Perhaps the current issue is another episode of the regulator failing to add cool water at all, but that couldn't possibly impact the temp displays from the first moment the heater is turned on. And yes, my pool tends to have high PH if you're wondering.

Your advice, as always, is wecomed!

Nick in Palm Springs