I replaced the fuel pump and a friend's O2 Pathfinder after replacing the fuel pump went to start and the security light on the dash on the left hand side stays on when the keys in the on position signifying that the immobilizer is not communicating correctly also the computer will not link to get codes. Looking for suggestions on where to start on diagnostics

Check all your fuses. Fuses 8 in the cabin and 62 under the hood serve most of the ECM functions, you may have popped one of them.

I should probably give you a little additional information. The owner of the vehicle was swapping the battery out and tried to put a battery that was oversized into the Pathfinder got a little aggressive with it and mash some stuff and just was a little too rough with it after I finished the fuel pump charge the correct battery and check the relays replaced the number 62 fuse you were speaking about under the hood by the battery which was blown and in doing so I guess wiggled everything around there but in doing so the correct setting or the correct reading from the immobilizer chip to the immobilizer system was gained and I got it started since then have been unable to start recheck relays under the hood and from what I understand the relay by the battery under the hood is the throttle body relay and the relay in the kick panel on the very back left by the rear liftgate is for the fuel pump itself. Both seem to be functional is there any other relays that I need to check. Fuel pump is coming on with the ignition switch in the on position for the primer function and then shutting off but I do not believe it's doing anything after that. Unable to get a communication between the code reader and the computer through the interface port. The only time that it did get an interface was the time that it started.

The ECM Relay and the ECM ignition input both have to be working if the fuel pump is priming, so I don't think you have a power problem. The fact that the scanner won't connect makes me suspect the owner may have damaged something while screwing around with the battery. One thing the whole Pathy/Frontier/Xterra line is infamous for is bad TCM's crashing the CANbus. Try disconnecting the umbilical from the transmission and see if the scanner connects again. If so, the TCM is shot.

I disconnected the TCM at the TCM the two connectors if that will suffice if not please advise I did not get a link connected or disconnected. I also took apart the power distribution box next to the battery check for any loose wires or obvious problems no issues found. Also checked the ground to the body at that area and no issue found. Next to the positive battery terminal is a bracket that holds three gray connectors also check those and found no issues. The red light that blinks for the security system on the left side of the dash under the vent is still staying on permanent in the on position for the key signifying the immobilizer issue

I'm not sure if I told you this already but initially the only time I got it to start I had replaced the fuse that was blown the 10 amp fuse that was blown in the power distribution box that you had said to check and also had swapped the relays for the I believe it's the throttle body that's also in the power distribution box after doing that immobilizer light was off when the key was in the on position and the car started ran short period of time and then started idling rough and died. No start since.

Still sounds like something is crashing the CANbus, or the ECM died completely and is incommunicado. The cluster will turn the security lamp on solid anytime the ECM stops talking, and for its part the ECM won't allow a start if it can't hear the IMMU. Have you measured your CAN voltages at the OBD connector to see if the bus is alive?

No can you give me a quick synopsis of test method please. CAN abbreviation stands for?

CAN stands for Controller Area Network, it's a variant of the I2C computer communication protocol but with push-pull data lines to minimize errors from electrical noise. Some form of it is used on all modern vehicles, and it's how your scanner talks to the OBD port. Pins 6 and 14 on the OBD port are CAN-H and CAN-L respectively, here's a good pinout for the OBD connector:
https://mechanics.stackexchange.com/que ... n-ignition

If your bus is healthy, CAN-H will read slightly above 2.5V and CAN-L slightly below 2.5V with the key on. The readings should still be there but at lower voltages with the key off, usually about 2V instead of 2.5V. Pin 16 must have battery power and pins 4~5 must have good ground for the scanner to connect. On most Nissans pin 8 should be ignition power. In addition, ohming across pins 6 and 14 with the battery disconnected should read about 60 ohms, that shows both the IPDM and BCM are connected to the bus (there's a 120 ohm termination resistor in each of those, in parallel they read about 60 ohms).