Can anybody help?

I have a '93 escort 1.4 with central fuel injection. The car has failed it's MOT due to ridiculously high emissions. It seems to be running very rich as the plugs are black and sooty. There is no mixture adjustment so I suspect a sensor or engine management unit problem. If any one else has experienced a similar problem and knows how to cure it I would greatly appreciate the advice as I don't know where to start looking. I also suspect there is a problem with the ignition relay, does anyone know where to find it as the manual does not tell you?