where i can buy a thermal protection device?

Ayuh,.... Is it the type that cuts the power, or the fuel,..??

Donno if the big box stores carry 'em, or not,...

If it's the electrical cut-off, 'bout any commercial Hvac supplier, or Electrical supplier oughta have 'em, in stock,...

If the fuel shut-off, the Hvac supply house would be the place to look...

How do you know it's not working?
Is it a main limit or a roll out switch?
An HVAC supply house that carrys Trane might be a starting point.

Could you post a picture of it? There are several types of thermal cutouts on the market. Some are even resettable

POST A PICTURE FOR GOODNESS SAKES!

Find out what cause it to go bad. thermal limits on a furnace are safeties, not operating limits. So there is a problem that caused it to go out.

The heating system stopped working. Then checked and the diagnosis box shows "thermal protection device needs to be replaced".

ying said:

The heating system stopped working. Then checked and the diagnosis box shows "thermal protection device needs to be replaced".

Click to expand...

The cause of its failure needs to be found and corrected.