My YS 640 constantly WAY overshoots the temperature I set it at, most of the time by at least 75 degrees. It can spend nearly an entire cook sometimes "coming down" to temperature, however, the temp reading on the Yoder itself is way off. I'm cooking ribs right now, have had the thing set to 250 for the past hour, it's been coming down from 290, and looking at my Thermoworks Signals graph, the thing hasn't even gotten higher than 236 degrees, it's been constant up and down as low as 200, it won't maintain a steady temp. I opened the door once to spritz my ribs, that's it. The temp probe is on the upper rack right in the middle. Am I the only one this happens to? Is my Yoder jacked up?
I think something must be wrong with your smoker. I used pit temp probes with my 640 once and it was very level and on point at 225 with 4 pork butts. Contact Yoder and let everyone know how it works out.