Most owners probably don't know that their vehicle has a crankshaft position sensor until the engine dies, won't start or starts running poorly. Then they find out they need a new one.
The crankshaft position sensor, typically mounted near or on the crankshaft, tells the engine computer how fast the engine is running. That's so the control unit knows when the spark plugs should ignite the air-fuel mixture and, in some engines, when to inject fuel. These sensors are used on virtually all engines that have distributorless ignition systems.
If the sensor fails, the computer won't know how to set the ignition timing, so the engine may stop running or refuse to start. It could also stall or run badly, possibly triggering the check engine light. Excessive heat is a frequent reason these sensors fail, and they also can stop working because of faulty electrical connections or wiring.
Some vehicles also have a camshaft position sensor that allows the engine computer to monitor the position of the camshafts (or camshaft), which open and close the valves, for more precise fuel and ignition management.
These sensors are not listed as routine maintenance items, but they often fail without warning. For that reason, some repair shops recommend they be replaced on high-mileage engines as preventive medicine.
Because a bad sensor doesn't always trigger the check engine light — and some of the same symptoms could be caused by other problems in the fuel or ignition systems — sensors should be tested with a diagnostic scanner or multimeter before deciding to replace them.
