ST170 management light keeps coming on

[Mr_C]

Hello people,

my dad's got a 2002 ST170 and despite having 3 or 4 lambda sensors, 2 cats and more diagnostic checks and resets than you'd believe, the engine management light keeps coming back on. He takes it in, they can't get a fault code off it, turn the light off and it's ok for a few day then, for no aparant reason, the light will come on again. My mum reckons it came on last time when she went over a bump, which might indicate a loose connection, although it could have been coincidence or she just imagined it...

Has anybody got any ideas of what it could be, and how to fix it?

thanks

Paul