I agree with all the above, but check the wheel bearing for end float, the Td5 (99 on )'fenders should have no end float at all as they have spacers between the bearing inner races to give a pre determined load. If there is any hub movement, the bearing has failed or the (peaned over locking) main bearing nut has come loose. This will give an intermittent fault and a sensor air gap fault.
I would also carefully tap the sensors in until they just touch the sensor ring, as they will be knocked back during rotation to give the correct air gap.
Having said all this, $800 is pretty good, as the last one I had to fit cost close to $1200. They do suffer from cable shielding failure and damage at the sensor end , but this is mostly evident on front sensors where the wheels are turned from left to right all the time, the wire literally can fatigue and break, giving an intermittent or total failure.

JC