Immobiliser - Engine no start

[Chumneywarner]

Hi all

E300D - W210 - 1996
Single button keyfob with flip out key (only 1 fob / key), IR receiver unit in rear view mirror with red & green LED's. No LED's in door handles.

Doors lock & unlock OK using the key fob. Turn key in ignition to start engine, engine cranks OK, but no start. Red & green LED's flash alternately during cranking & for about 30 seconds whilst ignition is switched on.

I have replaced fob batteries & re-syncro key fob according to owners manual numerous times, but no luck. I have tried a number of the other suggestions I have found on various forums too, but nothing seems to work.

Anyone have any ideas as to what the fault is & how to cure it?

Cheers
Garry