This code is for the transmission not seeing a crank signal from the crank sensor.
I attached possibilities and a flow chart for you to follow.
You will need to verify voltages at the crank sensor to see if the proper signals are at the connector. Do you have a volt meter to do some testing?
I also attached a wiring diagram for the starting system. I was not sure if you meant it would not crank or it cranked and would not start.
P0725 - ENGINE SPEED SENSOR CIRCUIT
Continuously with engine running.
This code is set when the engine speed sensed by the Transmission Control Module (TCM) is less than 400 rpm and the engine is running (As reported by the PCM over the communication bus) for 2 seconds. This DTC can take up to five minutes of problem identification before illuminating the MIL.
Related PCM DTC's present
Intermittent wiring and connectors
CKP signal circuit open to TCM
Sensor ground circuit open to TCM
TCM - Engine speed sensor circuit
Images (Click to enlarge)
Friday, September 18th, 2020 AT 4:15 AM