Nine times out of ten it is the exhaust regulator valve.(situated at the top left rear of the engine- it has one black lead coming out the top at right angles) You need a 'star' shaped driver to undo the two bolts holding it on. A blanking plate is only £4 from Vauxhall dealers.( take off the unit and just bolt on the plate) You may use a very small amount of extra fuel but its worth it!
Bazza, June 2014