Yes there are error codes. I'm just about to restore my laptop back to XP so I can use my diagnostics software again, this will tell me at least which sensor is not giving a signal to the abs unit. There are no connectors as such, they are all enclosed. Will have another fault finding session...
I am currently working through an ABS/ASC fault on an E39 BMW 528i M-Sport 1999. I have replaced all 4 ABS sensors and had the ABS module refurbished. The lights on the dash came on last year which is the reason I changed all 4 and had the module refurbed. The fault cleared for about 3...