Can anyone tell me what and where to check for problems with the Abs system when the dash light is on and where to find the abs control unit or how to diagnose the problem for the code, have removed carefully wheel sensors and cleaned but light still on

Disconnect the battery for 12 hours to reset the ABS ECU.

Make sure the ABS sensors have their probe intact and that the ring gears they pick up from are clean and the teeth are not damaged.