When you replaced the thermostat, was it the correct one? These engines are notorious for running "cold" in the winter.
the theremostat housing itself is a common cause. Was that replaced? When you put the stat in, did you index the thermostat so that it locks tight against the housing? In other words, stuff it in, then spin it until it seals against the housing as best that it can. If it's got a bleed valve in the stat (some do some don't), make sure it's pointing up (12:00), and that's all you can do.
When installing the thermostat, the two little legs need to make contact with the block. This forces the stat into the housing to seal it off. If it's not sealing against the housing, it will leak past and never warm up. I have had to throw an o-ring in behind the stat gasket (between the housing and rubber stat gasket) as a spacer to seal it up.
You will get a code for ECT if the engine is not up to operating temp (160+).