Temperature echelon imposed for heat conduction

Hi all,

I have a temperature condition imposed on space between [y1; y2]: T= constant ; and all others are adiabatic boundaries for the heat conduction problem.

Contours of total heat flux magnitude are not good at two zone around y1 and y2 ( attachment file); because of the temperature echelon imposed, i think.

Are you know other way to impose the temperature condition to prevent this error?

Thanks for your attention