Everything was working well on the car before the change of battery, so I think my problem must be key immobilser related, the central locking works, steering lock off, have tried both in Park and Neutral ( so not the XYZ lever), double and triple checked the battery connections ( they are very tight, and there is no corrosion) , the remote works, the key turns fine to start the car, but what has caused this to happen, for such a simple DIY thing - changing a battery ????????

I replaced the old battery with a new Bosch 95aH one, and followed the instructions below to the letter, (could not do the last one, item 10)

When I put the key in all the dash lights come up but engine will not crank and start !

1) to make things a little easier park the car with "Access Mode" selected ( lowest setting that the air suspension will go into) , then remove the key.

2) pull fuses 8 and 49 from the glove box fuse panel, this will stop any power surges to the sat nav computer and audio equipment, reconnecting a new battery with the fuses still in place has been known to cause damage to these devices.

https://www.fullfatrr.com/forum/topic69253.html

3) put the bonnet into service postition, unclip the gas struts from the wings then raise the bonnet to 90 degrees vertical and lock it into position by sliding the locking plate over the pin on the bonnets hinge.

4) disconnect the earth cable from the bonnet gas strut area ( V8 Petrol models only )

5) disconnect the negative battery cable, wait 20 minutes for the airbag system to discharge (the capacitors still hold some charge when the battery is disconnected and it takes around 20 minutes to fully discharge) NB the main reason for this wait is connected to the positive battery terminal there is an explosive pyrotechnic charge that cuts the positive battery wire if the car has an accident or crash.

6) after the wait, now disconnect the yellow explosive wire and the positive battery cable.

7) remove the two long bolts that hold the battery in place, carefully lift the battery out.

8. then carefully put the new battery in and bolt it back down, reverse the process, connect the positive battery cable and yellow explosive wire and then connect the negative battery lead.

9) refit fuses 8 and 49, and turn the engine on, then put the bonnet back down.

10) turn the wheel fully locked to the left, hold for 3 seconds, then fully lock to the right and hold for 3 seconds, this resets the steering angle sensor.