Due my MOT next week and I have a warning light on my dash. Not going to get a chance to get it fixed before it's due, Will I fail?

apparently it's a cam or crankshaft sensor, but seems to have no effect!! economy is as poor as ever, revs fine, flies along when requested, light just sits there shining at me!

btw its a R-Reg Vauxhal Vectra, would ask on Vauxhal forums but trust you lot more )

Cheers