I assume you have looked at the following link;
http://www.hexcode.co.za/products/gs-911/procedures/rdcDoes your GS-911 show the "Current ID: {7 digit-number}" for wheel set A, front & back?
Normally, when the battery of the sensor goes down a certain level, your dashboard should have shown the infamous yellow exclamation mark plus "RDC !".
According to the user-manual, only when "--" or "-- --" are shown plus the general alarm (red exclamation mark) there is a malfunction of RDC system.
Please be aware the sensors only go awake above 30 km/hour.
I would try option 2 in the aforementioned link;
- First take a ride for 5 minutes, ensuring that you have reached the speed above 30 km/hour for sure, for at least a couple of minutes.
- Upon return, the sensors should be "life" for at least 15 minutes when the speed is 0 km/hour, so hook up the trusted GS-911 and see if any error-messages are recorded.
However, as stated on the aforementioned link, this is *not* the right way to relearn the sensor. (!)
If you want to relearn the sensor-ID, I guess you must wait at least 20 minutes or so (after riding), and use the Emergency method (Option 2: waking a sensor manually), by leaving a substantial amount of air out as described in the aforementioned link.
(make sure you have the means to bring the tire back to its correct pressure).
PS: You can replace the batteries of the sensor;
http://www.youtube.com/watch?v=Mpjz0cDjnK4