why do you say that would kill the AC also, the AC condenser should work across the coolant temp operating range right? I'm still betting on air pocket, which would fool the sensor into thinking that it was below temp range. it's a sensor that is typically wetted and gets wonky readings in air space. even if the thermostat was wide open, the coolant temp would not be "below range" at any time. coolant is cold at startup right?Ahhh, ok, that means the t-stat is bad, stuck or sticking open. It won't heat to operating temp. This will kill the AC too.
Watch your temp carefully, I think you will find it won't heat up.
so does the sensor wait somehow until the truck is supposed to be warmed up? because obviously when you start the truck in the morning, on a cold day, even if the coolant is at 20 degreesF, that's still "normal" temp at startup. unless there is some delay in the system that throws a code if the truck doesnt reach operating temp after x minutes of operation?If the t stat is suck open the coolant won't get to normal range, so yes, it will be below range. The code happens when it sees the truck never warm up all the way.