Hi Tom, probably a few things to check. First of all, yes, try a good battery, if your other one has been sitting a year it may not be up to snuff anymore, or anyway it might need a charge.
Here is the info from the Sabertooth manual about the LEDs on the Sabertooth:
The green LED marked Status2 is used in lithium mode. It blinks to indicate the number of lithium cells detected. Also, when Status2 and Error are flashing at the same time, and you are experiencing no output from the motors, the unit is displaying low voltage mode. Charge your batteries first, and if that does not work, you will need a larger battery. Keep in mind that when a lot of current is pulled from a battery, the voltage will drop. The more depleted the battery, the worse the voltage drop. The Sabertooth will hold the unit in the error state for longer than the battery voltage drops. This is to stop the unit from stuttering and gives the user enough time to diagnose the flashing LEDs.
The red Error LED illuminates if the Sabertooth has detected a problem. It will light if the driver has shut down due to overheating or overcurrent.
Make sure also that the dipswitches on your Sabertooth are set correctly (there are dipswitches also on the TCB, but here I am talking about the ones on the Sabertooth itself):
See this link for a picture, but basically switches 1 and 2 should be off, and 3-6 should be on.
Also the Sabertooth 2x12 also needs its baud rate set by OP Config. This is done with the utility on the Misc tab of OP Config. Read the help file on that section (click the little "?" button) to learn more.
Finally, I assume you having the 2x12 and are controlling it via serial commands from the TCB. If you have the 2x12
"RC" version that is a different story, and you can read more about the correct setup for it at the same link I provided above.
Let us know what you find out,
Luke