Immobiliser led not functioning ..engine just cranks no start up..new battery in fob.. Removed centre lower dash bracket,,small control box with a red/grey spot colour wire just loose? checked all fuses.. self diagnosis says to bridge diagnostic plug and count flashes..no light showing on dash too count? seems immobiliser has cut off main electrics ?

well you hacked the IMMO
now you are in a pickle.
time to get the truck unhacked in a shop./
nobody can guess your hacks, or our proxy hacker actions.
nobody.

This light usually refers to the vehicle's Immobiliser System. Has this light just started to come on recently? Some Nissan models normally have this light flash when you walk away from the vehicle to show that the Immobiliser is active and working correctly while you are away from the vehicle.
If this light stays on when you try to start the vehicle it usually means the security system (the immobiliser) isn't recognising the key you are using. Are you having problems starting the vehicle?
If this is something it has just started doing try this simple fix first. Change the battery in the key fob for a good quality new one. An old battery can sometimes give a weak signal which the security system cannot read correctly and assumes it's an attempt to steal the vehicle.
Is the vehicle's battery in good condition and fully charged? Have you disconnected the battery recently? Some immobiliser systems need to be reset if the battery is removed for long periods.

Fob didn't work so the alarm immobiliser was armed and active when the locksmith opened the car - with predictable results.

Fob still not working - broken or needs new battery. Have you fitted a new battery into the fob?

The alarm immobiliser will probably remain armed until the doors have been locked and then unlocked with the fob.

There is a small chance the system can be fooled into thinking it has been disarmed by disconnecting the battery (overnight if poss) and then with the door open and the ignition key in and turned on, when the battery is reconnected it might be possible to start the engine...

The immobiliser works with a chip in the fob. This is a signalling device which, via a pick-up situated around the ignition lock, sends the signal to the engine computor which the allows the engine to be started.

run the fault codes to check for cam/crank position sensor
check fro broken timing belt/chain
check immobiliser operation-- flat fob battery)
check ignition control module

If it is not a new key fob, it sounds to me like the immobilizer is activated. So, all the lights on the dash are working now? Is there a flashing key on the dash lights? One of the resets that works is put the key fob in, and turn ignition on without starting it, and leave it 15 minutes. And try again.

if it worked fine before it is most likely either the new battery isn't charged or the connections aren't clean and/or tight

YOU NEED TO REPROGRAM IT

Don't know what size motor you have because you didn't post it so please pick the proper one:

This is for the 4 cylinder motor--


4-cylinder engine starter connections

1. Disconnect the negative battery cable.
   
2. Raise and safely support the vehicle on jackstands.
   
3. If equipped, it may be necessary to disconnect and remove the EGR pipe to access all the starter bolts.
   
4. Tag and disconnect the starter battery and control wiring at the starter.
   
5. If equipped, remove the starter bracket and/or heat shield.
   
6. Remove the starter-to-engine bolts.
   
7. Move the starter away from the engine and down to remove from the vehicle. Some starter may be installed with shims between the starter and engine. When the starter is removed, these shims may fall out. Do not loose them.
   

To install:

1. Install the starter and shims, if equipped to the engine. Align the mounting bolt holes and insert the bolts. Hand-tighten the bolts.
   
2. Tighten the starter mounting bolts to 30–34 ft. lbs. (39–44 Nm).
   
3. Install the starter bracket and/or heat shield, if equipped.
   
4. Connect the battery and starter control electrical wires to the starter.
   
5. If the EGR pipe was removed, install it.
   
6. Lower the vehicle.
   
7. Connect the negative battery cable.
   
8. Start the vehicle to make sure the starter functions correctly.
   

This is for the V6 motor--


6-cylinder engine starter installation

1. Disconnect the negative battery cable.
   
2. Raise and safely support the vehicle on jackstands.
   
3. Label and disconnect the battery and starter control wires from the starter.
   
4. If equipped, remove the starter bracket and/or heat shield.
   
5. Remove the starter-to-engine mounting bolts.
   
6. Lower the starter from the engine. If any shims are present, keep them for reinstallation purposes.
   

To install:

1. Install the starter and shims, if equipped, to the engine. Tighten the mounting bolts to 30–34 ft. lbs. (39–44 Nm).
   
2. Install the starter bracket and/or heat shield, if equipped.
   
3. Reconnect the electrical connectors to the starter.
   
4. Lower the vehicle.
   
5. Connect the negative battery cable.
   
6. Start the vehicle to make sure the starter functions correctly.
   

Check the fob battery because if the battery has gone then you can't immobilise the car immobiliser which in turn doesn't let you start the engine however it cranking. open the bonnet and remove the clips of car battery for a good 15-20 minutes and put them back and check your remote key and try to start your car. If nothing happens then let your car diagnosed by a experienced mechanic who is able to read out the ecu codes.