The gas ignitor has electricity passing through it, which opens the gas valve in the bottom of the oven. If electrical resistance stays low in the ignitor, the burner flame continues to heat the ignitor. But if the burner flame goes off, the electrical resistance goes up and the gas valve stops the flow of gas to the burner. Checking that the ignitor works well is very simple, the worst-case scenario being that if it is faulty, it will have to be replaced.
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Open the oven door--some doors can be pulled off their hinges to allow better access to the oven. Remove the bottom metal plate of the oven by lifting it up and pulling it out. The gas burner cover usually has a wing nut holding it in position: unscrew the nut and remove the cover. The ignitor--usually at the back--is either oblong or cylindrical, and it has a metal cover. It also has two electrical wires coming out of the back that are connected to two oven wires with ceramic wire nuts.
Turn the gas oven on. If all you can see is dull flame that's orange or red--or if there is no flame at all--the probability is that there is not enough electrical current to open the gas valve. In this case, you may also smell gas. Either way, the ignitor needs to be replaced.
Turn the oven on and look at your watch. Wait for the ignitor to start glowing. If it takes more than two minutes to glow, it needs to be replaced--even if the flame is fine and there's no smell of gas. However, if the ignitor lights with a healthy flame in a matter of a few seconds, your ignitor is functioning properly. Turn the gas back off, replace the gas burner cover and bottom metal plate in the oven, and close the oven door.