Mitsubishi 4g64 Engine Temp running Cold

Howdy out there, just replaced a 4g64 engine in daughters Galant Viento, when started it takes a while for the Temp to go up to 1/4 from cold, it almost reaches 1/2 on the gauge when idling and goes back to almost cold when driving on open road
I am thinking possibly thermostat or temp sender unit faulty, maybe ECU needs retraining.
Any ideas folks

Replace the thermostat first, it is almost certainly partly stuck open.

If the top of radiator isn't hot, then its thermostat. If rad hot, then faulty sender/gauge.

The hot when idling and cold on the road also point towards the thermostat which is definitely the most likely candidate anyway. You don't tend to "retrain" an ecu with regards to temperature either, they don't work like that.

there is almost always 2 temp senders one whom tells you with the instrument needle the temp and one whom tells the ecu what temperature the engine is .
the thermostat is definetly based on your info what should be exchanged , you could do testing with a heat seeking camera to see if the thermostat traps the heat when suposedly shut. but its probaby cheaper to use a new thermostat its all about cost efficiency , other methods might work also to check if thermostat is open when on open road hardly and warm air would come out of the heater in to the car compaired to when it reads warm.
as far as i know older mitsubishi are self learners and loose the codes when disconecting battery same on 95 toyotas even do you probly have never seen a check engine light come on on a old toyota.
you can check temps with scantool i can do it with my idiag android on the toyota with the full obd1 cable set i now have

The problem was the Thermostat, everything is good now.