Vauxhall Combo electrics

Hey I have just came across this problem but has been annoying me for a while. The fan does not come on when engine is hot etc, &
last week my van started to over heat and start drinking water like an elephant..

I found out that I had a hole in the water hose for the bottom of the water reservoir to the bottom of the rad..
I fix the problem and while I was at it I fitted a new stat etc etc.

This is when I noticed that my fan still wasn't coming on. Had a check with a volt meter and the fan is fine so problem must lie in the electrics somewhere.
Could some1 help and tell me how big a job this will be and if its even worth while changing the wires or if you know where the problem maybe. also when I run a diagnostic the car doesn't communicate with the device and it says there is no power, also I do not if some d**k head has turned the engine management light off.

help..
thanks dazza..