Eventually I found a 2005 Pontiac Montana SV6 instead of an Uplander, but I wanted to share something that might help other owners of this group of GM minivans. I had the dealer reduce the price to cover the cost of replacing the front wheel bearings because there were intermittent codes about the sensors. But since I wanted to rule out electrical problems first I cleaned the connectors and corroded battery cables. No change. The van always started up easy in even the coldest of days, so there was no reason to suspect the battery at first. But I did get conflicting reports from autozones tests, and it was several years old anyway.

Since I replaced the battery it has been almost a week and I am no longer getting the service messages. There might be other reasons for the service messages, but at least I can forget about these for now. Maybe the sensors can wait until I really need new front wheel bearings, as they are together.