Hi there and welcome!
Whatever you do please read the post below and do not throw the MES40 in the trash haha.
In short you can do a simple rewire (cut the ends off 4 wires and wire nut together to make 2 wires) and you can then run a PID controller.
The PID controller runs about $150 HOWEVER its like converting your current MES40 from a golf cart into a Ferrari!
The major strength of the MES units is the body and construction, they nail it here!
The weakness is the crappy cheap electrical connectors for the wiring and their electronics themselves.
If you rewire you bypass their electronics and controller and the MES plug will feed power to straight to the heating element.
The PID controller will plug into the wall and feed power on/off until the set temp is hit and then the PID will hold that temp right on or within 1-2 degrees of that set temp.
The PID controller has a temp probe that you drop into the MES so it can read the temps and cut power on/off to the MES.
Pretty simple and man it makes things awesome! No temp swings. Never fail to hit your temp. Tight control so no fear of melting fat out of sausage or bacon. Plus more benefits!
I know this may be a lot to throw at you but in a nutshell, you have an option that makes your MES way better than anything
Masterbuilt makes brand new and it's way less expensive then buying a brand new smoker especially when the MES is amazing with it's physical construction and temp holding characteristics :)
I hope this info helps, and see below for a very detailed write up on how to rewire an MES for using with a PID :)
Lately I've seen a number of posts about guys wanting to rewire their MES. I had wanted to post a quick and simple guide for those out there that are curious or those that simply need to rewire but need a little assistance. Well here goes. Disclaimer: When messing with electrical equipment...
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