Pool and Spa
Questions about Pool Repair? Ask an Expert Online.
You need to check the thermistor (temperature probe) that is attached to the plumbing on your pool somewhere in the equipment area. Either it is out and laying in the sun or it has failed and needs to be replaced. It can be checked with a meter if you know how to use a meter and interpret temperature, but most people will replace if it is old. If the temp reading is high then the heater will not come on as it believes there is an overheating situation.
I have attached an image of a probe/sensor.