Code 26- QDM circuit "A"

This is the code I received in my '90 Bonny 3.8L Vin C and the explanation is the above circuit. Can't find a reference to this circuit in my Chilton's to even begin to troubleshoot the problem.
Anyone have an idea as to which circuit or sensor I should be looking at?

Thanx

I had the same with 92 Bonneville. Try checking the fuse for the EGR power supply in the fuse holder under the dash by the right side A pillar. If it is blown you need a new EGR valve. I forget which fuse but check them all in that panel. Hope this works

Found the problem: The battery pooched a couple of weeks ago, so I guess it was faulty and not supplying the proper voltage to this circuit. Haven't had the code re-appear since new bat installed