Check your filter and gas supply first.

Start with the two most common causes: a dirty filter restricting water flow or a gas supply issue.

Pool heaters have a pressure switch that won’t let the igniter fire if flow is too low. If your filter is dirty or your pump is struggling, the heater thinks there’s a problem and refuses to light. Backwash or clean the filter, make sure pump is running strong, and try again.

If flow is fine, check that the gas valve is actually open (follow the gas line to the heater—sometimes kids or landscapers bump it closed) and that you’re not out of propane if it’s a propane setup. After that, it could be a bad igniter, a flame sensor, or a control board, but those are less common. Most of the time it’s something simple you can fix in five minutes.

If you’ve ruled out flow and gas, it’s probably time to call someone who carries spare igniters.