Just to narrow it down a bit, is this an issue that just started, or has this been the case since installation (or as long as you have lived there)?

Just started this cooling season.
In our first season in the house, we've always had the t-stat calibrated to back off -3* & once done gotten similar readings +-1* with a thermometer nearby.

Which was acceptable comfort wise.

But when this cooling season started, this one zone with the on-board temp sensor started showing higher than actual zone temp whenever the AC was triggered in this zone.

This kept the AC running continually to get to the desired setpoint...but the actual zone temp was wrong & therefore, made the AC run longer & the room colder than needed.

This post was edited by pbx2 on Thu, Jul 10, 14 at 15:02

Have you tried replacing the battery in your thermostat? That's the first thing the A/C guys always ask me is if the battery is still good.

Hi Arkansas_girl, my Trane t-stat is hooked up to a power source.

mounted on an inside wall or an outside wall?

@weedmeister -

Inside wall in a hallway. On back other side is a closet.

Across from it in this hallway is another closet.