What should I check first if my pool heater won't light?

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—backwash or clean the filter and make sure the pump is running strong. If flow is fine, check that the gas valve is open and you’re not out of propane. Most of the time it’s something simple you can fix in five minutes.