Lenovo T420 - Manual (Page 42)

Wire Voltage (V dc)
Red +2.5 to +3.2
Black Ground
• If the voltage is correct, replace the system board.
• If the voltage is not correct, replace the backup battery.
• If the backup battery discharges quickly after replacement, replace the system board.
36 ThinkPad T420 and T420i Hardware Maintenance Manual