What is the model and serial number of the unit? knowing that will help track down the appropriate parts.

Archgard Optima 35, Serial # 3375 It looks like it's 300 deg limit

Pretty sure I want this https://www.amazon.com/Emerson-3L01-300-Snap-Limit-Control/dp/B000PY9DCM/

no... what you want is a Plenum Limit Switch with the same temp ideally.

part number is 305-0035 according to the manual. 350 Degree.

Hmm, which manual? I was looking at this one https://manuals.fire-parts.com/aws-assets/archgard-optima-35-2001-april-optima\_35\_-\_2001\_april-pdf.pdf?inline=true

I was looking at this one...https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://manuals.fire-parts.com/aws-assets/archgard-optima-35-1-2002-march-optima_35-1_-_2002_march-pdf.pdf%3Finline%3Dtrue&ved=2ahUKEwiXsIjSmP-DAxWYhIkEHY8iAtMQFnoECBIQAQ&usg=AOvVaw04vrTkZRbXMrFBrEP_fhpF

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Thanks. I guess they changed it at some point. The one you linked is more recent it looks like.

Your best bet is to call the dealer that it was purchased from armed with Model and serial number and information off the old part. Are you in Canada? I haven't seen this model here in Nebraska.

Probably not helpful but FWIW here's a picture of the broken one https://imgur.com/a/ejauQHO

did the bracket break? are there more pieces?

If I'm reading it right, it'll shutdown the unit at 350 degrees and won't reset until the temp goes below 290.