Rainin EDP pipette won't sleep after 3 minutes

We have three of the Rainin EDP pipettes. All work normally except one won't sleep after 3 minutes. All have fresh batteries. All have a tip that matches the encoder plug. All charge on the rapid charger and trickle when full. All work on the 6v power supply with the battery removed - two correctly auto shut-off after 3minutes of non-use. I've swapped batteries, tips, encoders...and the one still will not power down after 3 minutes. The only thing different about the odd one is that it is black while the others are white. I can't find any answers in the manual or the web for the "Auto Shut-down." None of the F settings are related to power. Anyone experienced this and know the resolution???

Equipment: Rainin EDP pipette