When a homeowner calls with a complaint that his or her heater is not working, the cause could be a number of issues.
A bad temp sensor in the controller, a suction-side actuator stuck on pool suction, or that the heater is not turned all the way up. Similarly, when a pump is not working, it can be caused by a bad relay or time clock, or because it is losing prime.
It is not unusual for a customer to not know [what is really at the root of the problem], so ask as many clarifying questions as possible over the phone. Also, when you show up to the job site, think “over-prepared.” Bring parts and equipment for any repair that might be necessary for the situation the customer described.
— Brian Caudle, president, Clear Water Pool Repair,Katy, Texas