07 3500 chassis cab with DTC code 1401

[icexprt]

We have an 07 3500 that shut down while driving this morning and won't start back up. I scanned the computer and came up with a code 1401 "implausible engine speed signal received". Google search turns up virtually no info? Anyone have any ideas?

 

[icexprt]

I forgot the letter in front of the code... U1401. I replaced the crankshaft position sensor (ESS) but no dice! Truck still won't start. Had to tow it to repair shop.