Hello, here is what happens when unit is turned on. The small fan starts and runs while the ignitor is heating up. When the ignitor is ready the gas switches on and lites. However, as soon as the main fan starts the gas shuts off immedietly. The main fan will continue to run, and while it is, the ignitor will start all over and gas comes back on and lites again. The gas will burn for a minute or so and then go out again, and this cycle will just keep repeating. This unit is an older Janitrol maybe 10 or 12 years old. Any help?