What each symptom usually points to
A key that slides in but will not turn is most often the steering lock or worn wafers. A key that turns while the engine does nothing points to the switch behind the cylinder, the starter or the battery, and we say plainly when that belongs with a mechanic.
A cylinder that turns with no key, spins with the key in it, or lets the key out while the engine runs has lost its internal parts. That one needs a new cylinder, not a squirt of lubricant.
Why the new cylinder takes your old key
A replacement cylinder can arrive with its own pair of keys. Fit it that way and you carry two keys, one for the doors and one for the ignition, and the new ignition key needs its own chip programmed to the immobilizer, which is key work stacked on ignition work.
We set the new cylinder's wafers to the cuts on the key already in your pocket. Your chip stays in the same key, the car already recognizes it, and on most cars nothing is reprogrammed.
Ignition faults that are known on one make
Some ignition faults turn up on one make often enough to have their own page here, each with the model years, the repair and the dealer figure next to ours. Honda and Acura cylinders wear their wafers until the key stops turning: see Honda ignition repair. On 2011 to 2016 Volkswagen Jettas the lock housing fails, not the cylinder: see Volkswagen ignition repair.
Chevrolet and GMC trucks wear out the cylinder and the key together, and on older trucks a new cylinder needs a security relearn: see Chevy and GMC truck ignition repair. Chrysler, Dodge, Jeep and Ram models whose plastic key pushes into the dash use an electronic ignition node in place of a cylinder: see Dodge and Chrysler key won't turn.
The 2008 to 2012 Ford Escape and the older Focus fail in two different ways, covered under Ford Escape and Focus ignition repair. A Kia or Hyundai ignition broken in a theft attempt is its own job: see Kia and Hyundai theft ignition repair.
What the dealer does instead
A dealership often replaces the whole lock assembly from the parts counter, and on some cars that assembly comes with its own keys. That is how an ignition bill reaches $350 to $700, and how you can end up with a separate door key unless you also pay to match the door locks. If the car cannot be driven in, the tow adds $125 to $200.