2002 Vauxhall Astra cat sensors

OK, don't shoot the messenger. The o2 sensor is merely telling the computer what is going on so it can try to compensate by either adding or subtracting fuel to keep it at an ideal fuel ratio. It sounds like your car is running lean to me, po171, po174 are the usual codes for this. Check fuel pressure while at idle and check it with a load if safe to do so, more than likely with a load your fuel pressure is dropping really low. This is assuming that you have lean codes anyways. Post the actual codes and we can get a better idea of what's going on.