Assuming the starter motor spins but doesn't engage to turn engine over to start you could have a bad starter drive which is part of the starter assembly or possibly damaged/missing teeth on flywheel ring gear. Depending on year and model vehicle, some starters you can replace just the starter drive otherwise you need to replace the whole starter.
What do you mean when you say it doesn't engage? Is the starter spinning but just doesn't catch to spin the engine? IF that's the case it could be as simple as a loose starter or as bad as a damaged flex plate/flywheel. But it would most likely be the starter.
Some vehicles require "shims" to be installed when a starter is replaced so the Bendix drive will engage the pressure plate teeth. If the starter is too far away from the teeth on the pressure plate, the vehicle will fail to turn over.
Most often it is the starter and not the ignition.
Replace the starter
The starter drive has broken, replace starter.
If the starter spins but won't engage, then the Bendix is not engaging the flywheel. Remove the starter and test to see if the Bendix thrusts out, and check to see that the flywheel does not have chipped teeth.
thanks how many holding blots and where bad bendix (starter drive) if the spring in the starter bendix is weak or broken it will not engage the flywheel; but the starter will turn normally. bad thing is ,you have to pull the starter..good luck..
starter motor need overhaul
Toyota previa starter motor wont engage
1. To engage the starter with the ring-gear on the engine before the starter is turned on. IE to "pre-engage" it. 2. To turn the starter on once it is Engaged. This prevents/reduces damage to the gear-teeth on both the starter and the ring gear.
Starter selenoid no good
Aplly battery voltage to it, it should jump, engage.