Remove the igniter from the grill, or locate and open its electrode box; which method you use depends on your grill. Grasp the igniter and twist it while pulling gently. If it slides from the grill, then the igniter is likely a single component. If, on the other hand, your igniter is a button or dial that is part of the grill itself, your grill likely uses an electrode, which is contained in a breakerlike box somewhere on the front or side of the grill. Remove the electrode from its box, if applicable, by pulling it from its slot.
Examine the tip of the igniter or electrode for corrosion, rust or built-up grease, all of which prevent the igniter or electrode from passing a spark, which ignites the gas. If any of these symptoms are present, rub a piece of steel wool over the igniter or electrode's tip gently and sand away the debris. Replace the electrode or igniter and attempt to start the grill.
Examine the igniter's battery for corrosion or damage. Because the igniter works by passing electricity to the gas via a spark, each use of the ignition system drains its battery a small amount. Once the battery is low, the igniter may not spark, or may not produce a powerful enough current to ignite the gas. Replace the battery if it appears damaged, or if it hasn't been changed recently, by sliding the old battery out of its slot and inserting a new one in its place.
Fold a small square of aluminum foil and place it over the tip of the igniter or electrode, then insert the component back into its slot and test the igniter. It is possible that, due to repeated use, the component has become slightly damaged or warped and no longer making a connection when in its slot. The aluminum foil extends the length of the igniter's or electrode's contact point and completes the connection.
Locate the wire that runs from the igniter, if applicable to your grill's model. Examine the wire for a break or split area. If the wire has been split, strip a small amount of plastic from the split ends of the wire using wire cutters, then twist the two ends together and wrap the exposed portion of the wire with a liberal amount of electrical tape. If the wire has been tugged free of its pin, touch a piece of solder to a hot soldering iron, then touch the melted solder to the wire and pin, which connects the two.