2001 16v 1400 fabia 1 hi my abs light has come on, i plugged in a borrowed a halfords/rac code reader but its not showing any codes any ideas where i should start looking for the problem, is there a sensor on all 4 wheels or just the fronts thanks ken

is there such a thing as abs on only 2 wheels?

Generic FCR wont show all ABS faults. There are 4 sensors, one per wheel. Start with a proper code scan, otherwise it's just guesswork.