T905_O: DC Bus Undervoltage
On the VSODC, this fault would be displayed on the seven-segment LED as a number 15. Keep in mind, a 15 is an inverter fault, but does not specify any particular inverter fault. You will need to read the exact fault at the EcoNet thermostat or by looking for a yellow flashing LED in the upper right quadrant of the drive itself.
The Drive LED's will be flashing the yellow LED 8 times.
The DC Bus voltage is related almost exclusively to incoming AC Voltage. Verify the incoming voltage is above 187 VAC. This fault will likely occur in periods of dirty incoming power related to power flickering issues coming into the home. In some areas this can happen during severe thunderstorms or something along those lines where a momentary power cycle occurs. When this happens, the alert will be flagged and the unit will shut down. After the anti-short cycle timer has elapsed, the unit should restart and operate normally.
If this fault occurs three times in a period of time, the system may go into a short lockout and a A928_O lockout fault will be recorded. These alarms will clear themselves momentarily from the current faults but the system will remain locked out for 30 minutes unless a hard reset is performed. After the 30 minutes the system will restart normally. One indication that this is the case is you will see the A928_O lockout fault along with one or more T905_O DC Bus Undervolt faults time stamped less than 30 minutes ago.
You may also go to the outdoor unit status screen in the EcoNet thermostat and see the countdown timer says "locked". Again wait for approximately 30 minutes from the timestamp of the A928_O Locked fault and the unit should restart.
If the unit remains locked out and the voltage is normal, check the firmware revision on the VSODC. If the revision is before rev 41, replace the VSODC with a current one which will likely resolve the issue. Versions before revision 41 may be trying to reset, but cannot. To get the system back on line, remove the call for heat or cool, reset the alarm and reapply the call.
