Water temprature sensor

[Poor]

The engine check light comes on i have now looked up the code which is code four as above, the revs then raise and fall whether it is in gear or not of its own accord.
It is not an air leak and both sensors on the thermostate housing apear to be working ok.

I dont know if there are any more sensors, could some one please point me in the right direction or could i be looking at the wrong thing altogether?