The ignition lock cylinder is a mechanical part where the ignition key is inserted to start a vehicle. It’s mounted inside the ignition switch, the electrical component that locks the ignition and “reads” anti-theft coding in the key to allow the vehicle to start. Worn keys can damage “wafer” tumblers or pins inside the lock cylinder so that the key can’t turn, preventing the ignition from unlocking. The wafers themselves can wear down (so much so that in some cases the key can be removed while the engine is running), break or stick in a raised position, preventing the key from rotating. If the ignition lock cylinder is difficult to turn or the key can’t easily be inserted or pulled out, the key might be bent or worn. Having a new master key made by a dealer or locksmith -- not a duplicate based on the worn-out key -- may help. Some lock cylinders can be repaired by locksmiths by replacing or smoothing wafers. In cases where an ignition lock cylinder has to be replaced, the vehicle will also need new keys and new locks for the doors and trunk. Ignition lock cylinders typically last several years before problems develop, and they will likely be less of an issue in the future as more vehicles now are equipped with smart keys and push-button start that eliminate the need for a conventional key.