2013 Honda Civic 2DR EX 1.8-Liter M/T

I've been dealing with a persistent P0128 code (Coolant temp too low). I've replaced the thermostat and checked the coolant level and maybe the ECT is next.

Here's the thing: when the code reappears after having been cleared, the engine has only been running for about one minute. It seems like that's not enough time for the engine to get up to operating temperature so why does the code pop up so quickly?

I've had a friend check the live readings with his computer and, once the car's warm, the temp seems to be fine.

I think the computer is checking too early and generating the error code before the car has had a chance to warm up. Since I didn't get all the work done by the dealership, they are giving me the run-around even though the error code first appeared at 19k miles (just under 41k now but they say this isn't covered under the Honda power train warranty.

Anybody else have similar issues?

None of the P0128 threads that I've read have talked about this exact kind of thing.