I'm positive you can rewire and add a PID controller. Here is the link to a detailed rewire post I made:
https://www.smokingmeatforums.com/threads/mes-rewire-simple-guide-no-back-removal-needed.267069/
It won't matter what model your MES is the wires are basically the same. Only wire insulation colors and such may change or the fact that you might have to unscrew more stuff to get to the circuit board in the bottom of the MES but once there its still cut the ends off 4 wires and splice the proper ends to make 2 whole wires. Nothing more really.
Here is the post that will give you some more info on what the rewire for a PID entails:
https://www.smokingmeatforums.com/threads/mes-rewire-simple-guide-no-back-removal-needed.267069/
B
Brad Willingham
and
DanMcG
you an go a few different ways with PID controllers. If are familiar with them or mess with them already then you likely have extra parts/components and can build one for fairly cheap.
If you have to start from scratch or you don't really want to try and tackle building one on your own then I always direct people to look at the Auber PID controllers.
If you are starting from scratch then buying all the parts and a handful of tools to build a PID controller is about the same cost (or more) as just buying an Auber PID controller so hopefully this info helps in your decision making.
This one will cover MES30's and is on sale right now (Auber sales are rare): $120 -
https://www.auberins.com/index.php?main_page=product_info&cPath=14_28&products_id=72
This one will cover MES40's and MES30's (
good to have in case you ever want to upgrade to from an MES30 to an MES40) for $150 -
https://www.auberins.com/index.php?main_page=product_info&cPath=14_28&products_id=110
You can go any route you like with the various PIDs out there but this is a clear and simple path that will work very well for you.
Finally, if you are really getting in there and want to shore up your MES then it is a wise idea to replace the existing crappy electrical wire connectors with Hi-Temp Stainless Steel spade connectors, the following are an exact fit for the job:
Also it is a good idea to get a couple of orders of these EXACT safety rollout limit switches:
These switches are a little delicate so any movement or wiggle caused with the tab against the back of the switch WILL result in it overheating and melting down/failing at some point. When replacing the connectors on this switch it is good to have a few backups just for this fact AND just in case it fails anyways (... I've gone through 3 or more learning how delicate they are lol).
With the rewire job, the PID, the upgrade to better connectors, and spare safety rollout switches... your MES will run better than it ever did brand new AND you will be able to keep it running through anything short of it being shot with a cannon :)
Let us know what you decide and if you have any questions at all feel free to ask away :)