I bought a Toptak TP-200 code scanner "guaranteed to all 1996 and newer ODB-II vehicles". It works on my '01 Chevy but not on my '00 GMC. I keep getting an "ERR" error message when it's plugged into the GMC. The manual says that the type of error indicates the truck's battery is not powering the truck's computer. I don't see how that's the case since everything else with the truck is working fine. Any ideas?

check your cigarette lighter fuse mine was blown and wouldnt let me read codes

The topkick takes special software to read the computer. I know on my Snap-on Modis scanner, I had to purchase the heavy duty truck module in order to scan the truck. When I purchased the Tech 2 scanner, it has no problem reading the codes.