More then likely the BCM is what is keeping it from cranking . Anti-theft system light lit or flashing when trying to crank the engine ? Try putting gear selector in neutral to see if it cranks then , park neutral safety switch problem . Do you know what a wiring diagram is , need to view to see what all is involved in the cranking circuit .
STARTING SYSTEM PFP:23300 System Description EKS00F9V A/T MODELS Power is supplied at all times ? to starter motor terminal 2 and ? through 40A fusible link (letter. j , located in the fuse and fusible link box) ? to ignition switch terminal B. With the ignition switch in the START position, power is supplied ? from ignition switch terminal ST ? to IPDM E/R terminal 21. With the ignition switch in the ON or START position, power is supplied to IPDM E/R (intelligent power distribution module engine room) CPU. With the selector lever in the P or N position, power is supplied ? through A/T assembly terminal 9 ? to IPDM E/R terminal 48. Ground is supplied at all times ? to IPDM E/R terminals 38 and 59 ? through body grounds E9, E15 (all) and E24 (VQ40DE model). Provided that the IPDM E/R receives a starter relay request ON signal from the BCM over the CAN lines, the IPDM E/R grounds the starter relay and power is supplied ? from terminal 19 of the IPDM E/R ? to terminal 1 of the starter motor. The starter motor plunger closes and provides a closed circuit between the battery and the starter motor. The starter motor is grounded to the cylinder block. With power and ground supplied, the starter motor operates.
Your best bet mite be to have a qualified technician check it . Do you know what C.A.N serial data network is ?
https://www.nicoclub.com/FSM/Frontier/2007_Frontier/sc.pdf
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