There is no point changing parts at random. If the engine management light has been on, a fault code will have been stored. You need to get the fault codes read using diagnostic equipment. Vauxhall dealers are expensive, lots of independent workshops can read them for you and give you a print-out. Dont let them talk you into replacing parts, say you'll have to think about it.
When you have the codes, post them up on here and we'll see what we can do.
fred
December 2009