Vauxhall opel omega does not start when it's cold?

I have Vauxhall opel omega 2001 4 cylinders. It starts perfectly when it's hot. If it's cold the service indicator flashes when turning on the ignition and before cranking. It cranks very well. I changed the battery ad the plugs, but the problem stills. Some told me that it may be related to Fuel pump or Crank position sensor. Others told me may be immobilizer. I tried to use a fuel system cleaner but no use. Fot the immobilizer, Ithink It's not related to weather temprature. The last things I did for the car before the problem is Changing the engine oil and the radiator is slightly leaking.|||The service indicator does it look like a cylinder block with a lightning flash going through it? If so this is the Eng management light indicating you have a fault.You need to get a code read out to show what the fault is it needs diagnosing before you start changing partsIt wont be the imobilliser or it would not start when hot.It is probably the crank sensor but you need to confirm .Work on them Mick UK.