I have a brand new '21 Accord Sport 2.0T and it's coming up with code U0155.

It's been at the dealership for about a week now. They said they haven't found a fix yet.
They called this morning asking if they can put a couple hundred miles on it by having a tech take the car home and use it.

I haven't even had the car for 2 weeks and the first code came up a couple days after I took delivery.

Is this reasonable? They keep telling me with any brand new item coming off of an assembly line it's expected to have some imperfections.

 

U0155 is a problem with the F-CAN system. F-CAN is the fast network in your car, and I believe the gauge control unit is involved in that network.

Driving the car for a few hundred miles shouldn't reveal anything different. The dealership should be able to see what modules are on that specific network and verify if any modules are not communicating. The should be able to put an oscilloscope on the network to see issues messing up the communication.

You likely have a a bad module on that circuit or the wiring was damaged when made or during vehicle assembly.