Nick, it sounds like your pretty handy.

You should call tech support at Pentair and have them walk you through the diagnostics. Have a multi-meter on hand. One that checks voltage for AC and DC as you'll need it to check the circuitry.

Have your model number and serial number on hand when you call as it helps the tech guys find the info.

There are some many reasons for the heater not working. I would though make sure your filter is clean and you have good flow through the heater prior to calling tech support. A lot of heaters won't start or will be erratic due to low water flow-a dirty filter.

Thanks! I'm going to put a 10k resistor across the two thermistor pins of my control board with the wires from the thermistor removed. Apparently standard Pentair\Sta-Rite thermistors read 10 kOhms at 77 degrees, so a 10k resistor firmly attached across the pins should show a stable 77 degrees on the display. If the reading doesn't stay stable, then it's an aftermarket control board (about $300, it looks like) for me.

Water pressure isn't a problem here. The issue can occur even with the pump on the vacuum setting.

Cheers,

Nick

Update and solution for those having a similar problem. I pulled the wires from the thermistor sensor on the body of the heater and put a 10k resistor across the ends. In theory, the control board should have read something like 77 degrees but the temp display exhibited the same erratic behavior. To eliminate the wires as a source of error, I then hardwired the 10k resistor across the two thermistor pins on the control board. Alas, same problem.

That meant control board, unfortunately. I bought a new Pentair board on-line for about $310 plus tax. Took me all of about five minutes to install once the section of the heater with the board and display membrane is able to be removed and turned upside down. Guess what? Everything works perfect now. Only downside is that the temp readout on the Pentair board is less bright than what I had on there before, but it's not a big deal.

End of problem. I love those sorts of outcomes!

Nick in Palm Springs

That's a great outcome Nick. 11 years on a heater these days is pretty good. Those control boards do fail, I have replaced numerous over the years.

Understand those boards and the like are outside ALL day subjected to all weather conditions. It's basically like keeping your lap top out all day. At some point it will fail because of the weather conditions. Sometimes a power surge or lightning stike on the power supply can damage parts.

Lightning strike destroyed main GFCI and heater control board at my customer's this past summer.

Fortunately they have good homeowner's insurance. Even their WIFI in their house was destroyed. The strike was very close. It's a weekend home so luckily they were not there.