If its ony just come on I would say I is a wrong bulb so just check all your fuses to start with then check all your lights brake lights etc work and if not replace the bulbs.
Iirc the brake ligh switch beneath the pedal I believe is a common problem so if the above doesn't sort it purchase one of them.
Locate your knock sensor and check its tough inplace and check te wiring. Get a multimeter and test the wires to earth and check the resitance from two points as far from each other as possible on each wire to check if its broken. If its all ok you may need a new sensor.
As for the engine speed sensor the plug is beneath your inlet manifold so check its all ok and check for breaks in the wiring. If not you may need a new sensor. I did mine last week and a new part is £50 ish after vat from gsf.
