abs

Hi Have a 2.2 dci 135bhp on a 2005 plate ABS light is on, took it to my garage and he said its sensor needs changing on the front left. i need to buy the part but not really sure of what i am buying or the part number, can anyone help please. desperate as its booked into garage this thursday...

ABS lamp lights constant at startup and when the engine is cold. If I run a while and stops the engine, ABS light does not turn on again, when restarting the engine. At low outdoor temperatures occur this phenomenon more often than if we have summer temperatures What can be the problem ?