Check under the temperature display for lighted fault icons. If you notice a problem with your heater, you should immediately check the indicator lights. There should be five indicators. The first, resembling a lightning bolt, indicates a loss of flame. Another features a circle with a line, almost like a rotated "b." This is the fan icon. In either of these situations, contact a repair professional.
If the T-fuse icon, which looks like an inverted "s," is lit, the thermal fuse has opened, due to excessive temperatures. The fuse must be replaced, and the temperature regulated, for operations to resume. Similarly, the H-LMT indicator lights, when unsafe temperatures have been reached. If the condition clears, the problem will resolve itself.
The final icon is a circle with arrows representing water flow. This indicates a pressure problem resulting from insufficient water. When flow is restored, the heater will begin a new cycle.
Listen for knocking noises. These indicate that the heater is operating after the pump has closed off. Make sure all valves are open -- and if the problem continues -- it may be that the heater exchange is scaled. Turn off your unit immediately, and contact a service professional.
Observe the heater's patterns. If it is quick-cycling, or rapidly turning on and off, you may be experiencing insufficient water flow. The easiest solution is to remove and clean your filter and pump strainer. If the problem persists and your system is fairly new, it may be faulty wiring. Check the wires, if you are able -- or if you are more comfortable consulting an electrician, do so.
Check the electrical connections. If the heater does not come on at all, and the switch is clicked to the "ON" position, there may be a failure with the automatic ignition service. Secure all connections tightly and check for leaks, or a misplaced pump before calling in a repair person.
- A service professional can perform a series of five tests on your ignition module to identify the problem. It may be a simple case of miswiring, a bad transformer, or poor grounding.