I bought the 20W Neje Max4 when it came out a little over 2 years ago because I wanted a diode laser to go with my CO2s. Neje bragged about having the best laser modules on the market with the best warranty.
I don’t know about the warranty because I’ve never needed it but in June of this year lightning struck near my house and took out my TV, one computer and the microwave. I was running the Neje at the time and after the power came back on, I restarted the job I was doing. I few days later I noticed the cooling fans on the module weren’t working. The main board is fine and the fans work when powered separately leaving me to conclude the board on the laser module is the problem,
I decided to buy a 10W module because it will do finer work and I don’t need to cut much with it. That left me with a 20W that had no cooling. Obviously, the only solution was to run it at a minimum of 90% power until the magic smoke appeared and it quit working. So far I have put over 60 hours on it running it between 80% and 95% power and the darn thing refuses to stop working.
Attached is a picture of it “Taking a licking but keeps on ticking” (some old farts like me will know where that saying comes from). I put tape on the fans because I wanted to know if they ever worked. The module gets extremely hot but I regularly run jobs lasting over 40 minutes and it refuses to stop. However, I do pay very close attention when I use it because when it stops working I expect it to do so spectacularly.
The other picture is of the home built rails a use under the honeycomb bed. The are just the metal hanging ceiling tile rails. I got them from Habitat for Humanity’s Restore and they work great. The holes in them allow for airflow but I do have them raised 3mm above the metal baseplate which also helps.
I can happily testify to the fantastic quality of the Neje laser module.
Heat on diode emitters typically will shorten their life and they will loose intensity. No spectacular smoke singals from those, but the controller could.
There is another possibility, a broken wire feeding power to the fan.
@brewster is right, there are 4 diodes in there and they will not fail all at once. Just keep overheating it until 2 diodes go out and you will have the equivalent of a 10w diode module. 
I know - But I want spectacular! - lots of flame, noise, hair pulling, frustrated swearing; all set to music by the Amadeus Electric Quartet.
Followed by copious amounts of Scotch while telling everyone withing earshot of how surprised I was it failed.
I’m old fashioned that way.
I guess that won’t happen. It’s like a lightbulb. The diodes will degrade and just turn off at some point. Nothing else.
Oooooohh, what a bummer!
Pyrotechnics-Duct tape-Helium balloon-next storm…sorta thing.
You obviously need a K40 … 