I have a Dura Ace Di2 first generation and after 2000 km of riding have learned by my bicycle repairman that i have to pay about $160 to replace an electronic relay which I have to buy with the battery pack holder ( which I don't need). The problem which forces me to proceed with this totally unexpected surprise is caused by the fact, that when I change speed on the back derailleur my chain on the bigger front plateau the minute i press to change speed the system changes the chain from the larger to the smaller plateau without. I am told this is not covered by Shimano really not satisfied that after paying so much money for a state of the art system i get such an unpleasant surprise! .

SOURCE: Topic : "Shimano DuraAce Di2 ST-7970" and the

It is apparently critical for the success of the pairing operation that initial power-up of the component to be paired be done cleanly, with little or no contact bounce when the battery is inserted. Since this is done with low power high frequency radio signals, interference and reflections can be an issue. If you have extraneous gear such as metal water bottle brackets mounted near the system controller / battery pack, remove it for this operation. Do not let anyone play with R/C toys within 30 feet while you are doing the pairing, and turn off Bluetooth devices to be really sure.

1. Remove the batteries from all components of the system except the SC-7900, including the heart rate monitor and the speed sensor. Wipe the battery contacts of the shift system with a cotton swab moistened with isopropyl alcohol (get straight alcohol if you can, rather than the 70% rubbing alcohol mix). Clean the battery terminals with a clean paper towel moistened with isopropyl alcohol.
   
2. Click the shift buttons on each lever, then use a piece of wire or suitable tool to short the battery contacts of the shifting components (NOT WITH THE BATTERY INSTALLED!) for 10 seconds. This will ensure that any power bypass capacitors in the system are fully discharged and the circuitry will completely reset when the battery is put back in.
3. Set the SC-7900 into pairing mode as described in the manual (latest version can be downloaded here). I suggest removing the SC-7900 from its bracket and parking it someplace where you can conveniently grab it when you reinstall the battery in the shift system.
   
4. If there is a power switch on the system, turn it on first. Push the shift system battery into place smoothly and hold it. If you can lock the clamp in place while holding the battery so it does not slip even a little bit from the contacts you may do so. Do not give it a second push "for insurance" at this time; this may generate a contact bounce.
5. Immediately press the Select and Set buttons on the SC-7900 to put it in scan mode and hold the SC-7900 about 1-2 feet (30-70 cm.) away from one side of the bike. The pedal on that side should be in the lowest position to minimize any effect of radio signal reflections from it.
   
6. If the pairing does not complete, remove the battery, then disconnect the electrical cable from the rear derailleur. Repeat step 4 and press "retry" (the Select button) on the SC-7900 to repeat the scan. If this works, the front and rear derailleurs are separately paired devices. In that case, remove the battery again, reconnect the rear derailleur and repeat the scan.
7. Once the shift system is paired, you can pair the heart monitor and speed sensor. Note that all of this must be done in one pairing session. Do not exit the pairing mode until everything is paired, or you will have to start over.

The graphics shown in the SC-7900 supplemental manual imply that the right and left shift levers are separate devices, but they are powered from the same battery and connected to the system at a single point. While Shimano's online documentation on this is rather sparse, the Di2 7900 system description says the system logic and communication circuits are inside the front derailleur. I do not expect step 6 will be needed. If you go through step 6 and it doesn't work you may need to disconnect the shift levers individually and try again. Once a device is paired, you should not have to disconnect it or remove power from it to continue the process with other devices.

If none of this works to pair your shifting system with the SC-7900, then the wireless circuitry in the shift system is defective (as I mentioned previously, this is probably in the front derailleur). Even if it is out of warranty, you may be able to get a warranty replacement since it is a newly discovered defect that could have existed from the factory. You will probably have to talk directly to the Shimano service department for this.