When I had a sensor fault in my 2000 ABS/TCS ST1100, I discovered something that may help. I found this out by accident one day and I wrote it down and then repeated the process when I got my replacement sensor to see if it really did work. It seemed to.
The ECU will record a fault if the sensors stop working during the course of riding.
ie - they have been working, but then they stop working - so a fault is recorded. However, if it is a faulty sensor, the system will never store the same code again.
Note that the same error code is stored if after arming itself, the system detects that one wheel is rotating faster than the other (eg wheelies, very bumpy ground, putting bike on centrestand)
The ECU will NOT record a fault if the system fails to arm itself when you start out.
My experience is that the ABS and TCS warning lights come on and stay on when one sensor isn't working. Because the system hasn't armed itself by verifying that both wheels are turning at the same speed, it cannot check if the sensors are working or not. One faulty sensor, or problems with the connectors / wiring loom will cause this to happen, and there will be no recent fault code stored in the ECU. Only historic ones will show up. So - if you get a faulty sensor, you will never see the sensor error code again after the code has been cleared from memory, or after other (possibly false) errors have over-written the stored errors.
So - without an error code, how do you check if a sensor is faulty ?
Before doing this, make sure that you have recorded the 1 or 2 codes that have been stored - because you are about to clear them from memory !
Try to retrieve the problem code as before (yes I know you've done it already. Do it again).
• Clear the problem code (yes - even if there wasn't a fault recorded)
• Do NOT turn the ignition off.
At this point the lights look just like they are when you turn the ignition on - but this is not the case. Only if you have gone through the above procedure can the following test be carried out.
• Spin the wheel as fast as you can and watch the ABS warning light. If it flashes, the sensor is working. (nb - you will need a mirror or a friend to check this for the front wheel - the light only flashes while the wheel is in motion. The back wheel is easy, you can start the engine and slip it into gear)
And of course, a useful check is that the two sensors are interchangeable - front/rear. They are a bit difficult to remove, and I would suggest pulling a bit of string through when you remove the sensors to make it easier to thread the cable afterward. The front sensor cable clip requires a knack to get it unclipped. I don't have that knack. It came apart after much prodding and fumbling.