Troubleshooting gas heaters can be tricky because many times the trouble is not actually with the heater.
It is a good idea to backwash the filter before working on the heater.
Make sure the pump is running with the main drain and skimmer valves open. Set the gas pressure to the proper water column pressure for the individual heater, measured with the gas valve ON.
Electrical connections must be tight and installed according to code. Make sure that there is power to the heater, and that the thermostat is set for heating.
On a millivolt heater, one common problem is the inability to keep the pilot lit. This is often true when starting the heater after the pool has been closed for the winter. Insects or debris can block the flow of gas. It is therefore important to clean the pilot assembly.
If the pilot lights, but goes out after the gas knob is released, remove the control/limit circuit wires from the gas valve terminals to avoid a short to ground.
Light the pilot. If it won’t stay lit, connect a multimeter to the pilot generator leads and test the DC volts. It should read about 500 mV. If the pilot generator is less than 400 mV, remove the pilot generator leads from the gas valve and connect directly to the meter with the gas valve knob held down and the pilot on. If the pilot generator is less than 600 mV, replace the pilot generator. If it is 600 mV or above, replace the gas valve.
Other items to investigate:
– Check external wiring and switches.
– Check the gas valve.
– Check the control/limit circuit (this will involve a jumper wire and multimeter).