Trouble code 18010 Power supply (B+) Terminal 30 - Low voltage

I recently noticed the check engine light was lit in my car. So I bought an engine scanner to see what the problem was (I thought it was the MAF). But the scanner read: 18010. In the the manual, that came with engine scanner ( I don't have my car's owner manual), the definition given for this code was: Power supply (B+) Terminal 30 - Low voltage. But that doesn't tell me much. Could anybody out there, please let me know what this means. After scanning the engine, I erased/reset the problem code and the check engine light is now off, even after a good ride.

Also, my car is a 99 A4 2.8

Thanks any thoughts would be great!!

I'd like to fix this myself if possible.