The gas supply and thermocouple are your first checks.

Yes, most pool heater ignition problems come down to gas or the thermocouple. Start there before you call anyone.

First, confirm the gas is on. Sounds dumb, but it’s the most common fix. Check the shutoff valve at the heater and the main supply valve (or tank level if propane). If it’s a propane tank, make sure it’s not empty — an empty tank won’t flow gas, and the system won’t ignite.

Next, look at the thermocouple (or thermopile). It’s a small sensor near the pilot that tells the gas valve to stay open. If it’s dirty or failing, the heater will try to light and then shut off. Clean it gently with fine sandpaper or steel wool. If that doesn’t work, replace it. They’re cheap and easy to swap.

If you have an electronic ignition, check for a spark when you call for heat. No spark? The igniter or control board