How do I know if the spa heater is on from my phone

could be the heater is defect, the thermostat is defect or the thermometer is defect. Try to rule out 2 of these and you know what is wrong. Or het heater just does not get any power?

turn the valves so ALL the water goes through the heater and back to the pool

What????
Portable Spa with its own components/equipment below (electric heater) or do you have a separate equipment set installed away from the spa. Or is this a swimming pool? With a gas heater?
Let us know more about what it is you have that has definitely malfunctioned!Make, Model, Serial numbers. And if it is a portable self contained spa, not just the spa maker, see if you can isolate the control system & its specific information.

Since I don't know exactly what model you have I can only suggest you call the Pentair technical support at 800-693-0971 andthey should be able to help you figure this issue out.

I hope this helps.

unless plug is bad u will need to check the motor or heater one of them is pulling high amps heating up plug--oil motor may help--but will need to check the amperage on the motor and or if heater is cause if no amp probe --when system is cooled plug in and turn down heat setting let motor for a while if plug heats up or motor goes off on over heat know problem is with motor or plug-if no change now add heater if plug gets hot and turns off when heater is applied know heater is problem

Try the owners manual for these pool heaters. They have a detailed troubleshooting guide that should let you know what you need to do.

http://www.backyardcitypools.com/images/bwp/Max-E-Therm-Manual.pdf

It is just letting you know it is in the spa mode rather than the pool mode. Your OK, but it is good to ask when not sure. If more people did this I would probally go out of busness.

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Here is the Mfg Link so you can trouble shoot your heater.
RayPak