Yep like Winter said.
On the MES the Controller is on the top and is where you press the buttons and see the temp and other readouts.
The MES controller is one of the two most common reported failure points.
Masterbuilt will send you out another in no-time without question most of the time.
A PID Controller is one that you buy as a 3rd party part. It would replace the MES controller and does the same job as the MES controller BUTholds temperature much much closer to the set temp of the smoker... like dead on with deviations of maybe 2-3 degrees if it deviates from the set temp at all.
PID stands for the P, I, and D which are the tuning parameters that are set that drive the controller to manage and hold temp. (Don't worry about the technical details of P-I-D at the moment).
The MES Controller intentionally causes temp swings because it wants temp to go up for a long enough time and usually over the set temp to help ensure that the heating element is burning the wood chips to produce smoke.
The PID Controller cares about no such thing and with an
AMNPS there is no need for a controller to care since temp and smoke are managed separately of one another :)
Now if going with no PID controller modification, I would buy any MES40 that was a Gen 1 or or a Hybrid Generation that has the vent on the top left (not side left) of the smoker and does NOT have the Gen 2 guts.
I would go with the least expensive and most plain one out there like this one. Academy usually has the best prices I have seen on MES smokers.
https://www.academy.com/shop/pdp/ma...gIqcfD_BwE&gclsrc=aw.ds#repChildCatid=4903505
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If I wanted to go the PID route i would buy the cheapest MES40 I could get my hands on with my preference being that the vent was somewhere on the top of the MES, not on the left hand side of the smoker. When I say left hand side I am meaning that if you leaned up against the left hand side of the MES box/smoker you would cover up the vent hole. Where top is that you could only cover up the vent hole by setting something on TOP of the MES.
I have found great deals on used ones on craigslist.org so keep an eye out. To use a PID controller a simple rewire is done to the MES so even if the used one has controller problems or wire connectors are corroding that all gets fixed very easily to prep for the PID controller so everyone wins. They may need a little cleaning but as long as the element works then it doesn't matter what else may be wrong with it :)
I hope all this info helps :)