Reference Mark Sensor
 

Ok, what is it and what does it do? Most importantly, can I run the car without it since it broke off and cost $135 to replace!

Depends on what one it is.

The refrence sensor tells the DME where the motor is when cranking. Since there is no cam sensor on our cars, this acts as the same. Once it gets the signal, it looks at the other sensor for it's position and can tell then where the motor actually is.

And no.. you cannot run it without one... well.. you shouldn't be able to.

Did the reference mark break off or has the sensor gone bad? The clearance between the mark and the sensor could also be out of adjustment giving the DME a bad reading.

Either way the car will not run without a good reference mark reading (used as a reference for ignition timing and the start of the injectors firing).