Have had a long running problem with the ABS light coming on - usually when driving very slowly. It would remain on until I turned the engine off. On re-starting it would be on for the first 100m,
then after self diagnostic checks, go off. Would be OK for a few days then the same problem would re-appear. I checked all sensors all read 1,050 ohms. Eventually I gave up and took it to a Citroen garage. For £19 they found it was a poor signal from one wheel sensor (rns).
Took the appropriate wheel off, checked the wire and sensor for physical damage - all OK. THEN brushed the toothed wheel clean. No further problem (for 4 weeks now). Can only assume there was some build up of "iron" between some of the teeth which resulted in an abnormally low signal. This would correlate with the light coming on when driving slowly.