In COLD weather the ABS light stays off. As soon as the weather warms up, the light comes on. My mechanic ( a bright guy ) says the PCB board is failing on the ABS ECU, and in cold weather, resistance is lower, hence no light. FWIW: the volvo dealer (!?!) said the code was the right rear wheel sensor (see bay13) and basically this indicates the ABS ECU rather than the wheel sensor. Hello cold weather!
Comments welcome!