Low gears not selecting

Hi, i have an "M" reg P38 2.5 diesel. Alls well but today I pressed the button on the dash board which selects low gears and nothing happened except the led on the switch started flashing on and off ? Any ideas what the problem is or maybe. It was working fine two weeks ago.

regards

Assume it is a Manual then....

So Main Box in Neutral, Press Button nothing happens....??

My guess would be the Transfer Box Selection Motor....or even the TB ECU - they have been known to burn out quite readily.

A quick check on diagnostics will show any codes, but the most common thing is the motor.