I has one of those little plug-in ODBII devices in the van. Monitors whut is going on and declares if and when there is a fault. It was running (presumably) fine until I went fur the new transmission. It was lying on the passenger seat when I picked the van up again.
Now doesn't work on my cellfurn. Garage says they "had trouble connecting their device too".

How/why could the system break down just from unplugging a device?
Whut I do to fix it?

disconnect the leads from the car battery, hold the battery leads together for 30s. then let it sit 10-15min before reconnecting them.

Can't guarantee it'll work, but sometimes that'll break the computer out of it's funk.

Other possibility is the shop f'd up the connector. You may want to have a different shop take a looksee.

I don't trust mechanics with the "it was like that when you brought it in." Not to say that it couldn't be some other problem tho - depending on age/make of vehicle it could be iffy wiring. Still, try a different mechanic.

GrandCherokees/Durangos can be all fuxxored in really bizarre ways when the wiring to the drivers side door goes bad.

Neither do I, Clara 🦄 .