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It won't relight because there is air in the line since you ran out of propane. The safest option is to keep power cycling the unit. Turn the power off once it locks out due to failed ignition then wait a minute and turn it back on, keep doing this until it fires up. It can take a long time to do it this way. The other option is to close the gas shut off valve to the unit then disconnect the gas line then turn the gas back on until you can smell the gas (no open flames anywhere in the vicinity) then turn the valve back off. Then reconnect the line turn the gas back on (put slightly diluted dish detergent on your connection to check for leaks) then cycle the power again after waiting 10 or 15 minutes. The second option is how we would do it but if you aren't comfortable with disconnecting the gas line then stick to the first option.