The F3 code appears on a Glow Warm Boiler’s display when the fan is unable to spin fast enough to launch ignition. In other words, the fan is rotating at a speed that is lower than it should be. This may be due to a mechanical fault or loose wiring. Not infrequently, the problem is caused by cold, when the condensing pipe freezes up and water remains in the air duct or the flue. The ice blocks the fan and won’t let it rotate at full speed. In any event, it is strongly recommended that you trust a trained technician to fix the problem instead of trying it yourself.