Honeywell dt90-temp stuck

[Quietlife]

I've just replaced my Honeywell dial thermostat with a dt90. Very simple like for like replacement. However, the DT90 is now constantly showing 19 degrees as the room temperature. It never varies regardless of whether I turn the heating on or off.
Does it need calibrating somehow (I looked at the installers guide but nothing in there seemed to be applicable )
Thanks

 

[D_Hailsham]

Can you post a pic of your DT90's display?

The stat does not need "calibrating" but there is a parameter (To) which allows you to correct for differences between actual and displayed temperatures.

 

[Quietlife]

I've left all the windows open and it still doesn't alter. The room was definitely colder than 19 degrees

 

[D_Hailsham]

Is it a new stat, in Mfr's packaging with all literature?

There is no light in the bottom right corner, which means the stat is turned off. Press the button at the bottom and hold down for two seconds; the light should come on.

What was the old stat and how was it wired? How is the new stat wired?