Some times the ABS sensor on each of the wheels can get a build up of brake dust on them, this can cause the sensor to read incorrectly and give a fault read out, or maybe one of the sensors are misalined, I'm just going on what a Subaru service agent told me when I took my daughters WRX in with the same problem. you could try removing your wheels , and trying a can of brake cleaner on each ABS sensor, rinse of with water.
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Some times the ABS sensor on each of the wheels can get a build up of brake dust on them, this can cause the sensor to read incorrectly and give a fault read out, or maybe one of the sensors are misalined, I'm just going on what a Subaru service agent told me when I took my daughters WRX in with the same problem. you could try removing your wheels , and trying a can of brake cleaner on each ABS sensor, rinse of with water.
The abs relay may be bad