Davidm and vic are right about the ABS module. It is a defective part that Volvo should replace at their expense. You will end up paying $500-600 for the repair. Remember, this is a safety related issue because when the ABS light is ON, you DO NOT have functioning anti- lock brakes, so brake accordingly. You should contact Volvo customer service and tell them that Volvo should replace this part on hundreds of cars, at thir expense. Also contact the NHTSB and tell them that this is a safety issue that volvo need to address.