abs light on all time?

I have a citroen xsara 2001 2.0HDI 90hp and the abs light is permanently on, also the brake pad wear light goes out for a second and then comes back on. The ABS is working, I can feel the pulsing, there are no fault codes when interrogated with diagnostics, all fuses are fine.
The car does not even have wear sensors on the brake pads there are no wires.
I am stumped on this one, can anyone help?
Thanks
Peter