A crank sensor is a component used in an engine to monitor the position or rotational speed of the crankshaft.
This information is used by engine computer to control ignition system timing and other engine parameters. Before electronic crank sensors were available, the distributor would have to be manually adjusted to a timing mark on the engine for this same purpose.
The crank sensor is often used in combination with a similar camshaft position sensor to monitor the relationship between the pistons and valves in the engine, which is particularly important in engines with variable valve timing.
Because the two sensors work in unison as a part of the same system it is not unheard of for both to go bad at the same time.
However, it doesn't ALWAYS happen that way. The mechanic will read trouble codes from the on board diagnostic (OBD) system to determine which sensors are bad. Because these sensors work in unison it is possible for just the crank sensor to be bad but its in-operation can cause the cam sensor to show a bad code.
When we see both codes we often replace the crank sensor then reset the codes. Often, that will cure the problem without the cam sensor needing to be replaced. The trouble code stating the cam sensor is bad was simply a phantom code related to the bad crank sensor.
As for the life of these sensors, it isn't unusual for them to begin to fail around 100,000 miles or so. However, that doesn't happen in the majority of cases. A good 60% of these sensors will never be replaced during the life of the vehicle. The fact that the majority of these sensors make it well past 100,000 miles is why they aren't included in regular maintenance. The Oxygen sensors are the only sensors with a fairly predictable lifespan so they are the only ones you'll usually see included in regularly scheduled maintenance (if any are included).
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