Theory Of Operation
WARNING: This page is about a different car, the 2015 Fiat 500. However, it is still accessible from the selected car via links, so may be relevant.
The Transmission Control Module (TCM) constantly calculates what the output shaft speed should be in any given gear based on the input speed sensor, output speed sensor and the current gear state. This DTC indicates a C1 or B1 clutch did not engage.