Can OBD-II scanner detect the error if crankshaft sensor is down?

Sometimes the computer won't pop a code for the CPS but there's problem to it and will prevent the engine from starting therefore its possible-sample codes for CPS below

P0335....Crankshaft Position Sensor A Circuit
P0336....Crankshaft Position Sensor A Circuit Range/Performance
P0337....Crankshaft Position Sensor A Circuit Low Input
P0338....Crankshaft Position Sensor A Circuit High Input
P0339....Crankshaft Position Sensor A Circuit Intermittent
P0340....Camshaft Position Sensor 'A' Circuit #Bank 1 or Single Sensor#

Try to read the data on your scanner and read the rpm data if it stays at 0rpm check voltage at the crankshaft position sensor if you have power then replace the sensor

The question is WHY you are inquiring about the crankshaft sensor. For your information, if your vehicle is equipped with the 3.0L OHV V6, it is probably a CAMSHAFT POSITION SENSOR & SYNCHRONIZER that needs replacing, if it exhibits the following symptoms. 1. You hear a squealing type noise that is extremely similar to, and often misdiagnosed as a belt squeal. 2. The engine runs rough, sometimes stalling. 3. If you do happen you get a P0340 CMP circuit failure code. Again, this is only common to 3.0L OHV V6s. If your vehicle is equipped with the 3.0L DOHC Duratec, disregard everything I've just posted. If you do need a new cam sensor, go to your local Ford dealer parts department to purchase it. (FORD base part numbers, 6B288 & 12A362) Hope this helps. To add to your question, ALL Taurus engines use a timing chain. It would help if you post which engine yours is equipped with. To add further to my post, if you do purchase the two aforementioned parts, the sychronizer needs to be timed the way a distributor would, during installation, failing which would result in ignition and fuel injection timing being out.