I would assume that code means the ECM isn't seeing the normal voltage drop when it sends the signal to activate the grid heaters? IE it is trying to alert you that there is an open circuit in the relays or heaters (as it would be when disconnected) and they aren't working...

 

Just to confirm what JBody has said... I had trouble this weekend with my charging system. First I heard a loud squeel and few minutes later the volt meter in the truck went all the way to the left. I picked up a new alternator and changed it out. While I was changing it out I noticed acid everywhere under the passenger side battery. I'm guessing the squeel was the seal in the die hard platinum giving up the ghost. I started the truck with just the driver side battery and all was good on the dash voltmeter and measuring at the back of the alternator and at drivers batt. I went and bought a new battery for that side and hooked everything back up and the trucks voltmeter was reading around 12v so I measure the voltage at each battery and found that the drivers battery was only getting 12 volts and the passenger side was getting 18v and was already hissing.

The fix was to take apart every connection on the positve cables of both batteries and clean them up real well and I also checked for continuity throughout all the way from the altenator to the driver side positive terminal. There was very little corrosion on any of the connectors but I guess it doesn't take much to cause a problem. After everything was cleaned and put back together all was normal again. knock on wood. Just thought I would share my experience. usually I'm not that lucky that it could be something as easy as cleaning some terminals.

 

I had a similar problem. Two new batteries then I noticed one of them was swollen and leaking. Replaced that one and it started to do it again. Long story short, crossover cable was loose putting most of the charge in the passenger side battery