What should I check first for pool heater ignition problems?

Start with the gas supply and thermocouple. Confirm the gas is on — check the shutoff valve at the heater, the main supply valve, and the propane tank level. Then inspect the thermocouple or thermopile; if dirty, clean it gently with fine sandpaper or steel wool, or replace it if needed. For electronic ignition, check for a spark — no spark means the igniter or control board may be the issue.