Hi, I should check how the system is handling the idle: potentiometer on the throttle body ? Electronic air valve ?
And check first those components: position, move free or without play, use a multimeter to check that the potentiometer is well giving a value on all the range of movement, etc ...
Also check the cabling form the temp sensor to the ECU.
If nothing found then back to dealer to verify the idle value set in the ECU and if correct check the ECU
Unfortunately without ECU reader you cannot do much more
