Is that Celsius? I’m not 100% sure to be honest haven’t tested that high. Avoid dry firing.
If the MOSFET overheats and burns out, it’s toast… once the magic smoke comes out of the bottle, there’s no putting it back in.
Don’t have specs on temperature tolerances for MOSFETs, but 39°C isn’t THAT hot. Body temperature sits at about 37°C so it’s hard to believe 39°C would affect function.
Humidity would be more of a concern… moisture and PCBs with soldered components don’t play well together.
Got my last few for the shelf from here LH AUG MOSFET – MASTR BLASTR
Vector ones also fit with a knife mod to split the JST plug. Same thing besides the plug
So now fires a few shots the stops , unplug battery then replug then fires a few shots , stops , when it does this theirs no prime function ether [no power] have I rooted the mosfet ???,
Hi , I just pick up 2 LH AUG mosfets $10 each .
From master blaster 
Ah have a look at the shot counter left front of the gearbox. Sounds similar to trouble I had. A puny wire fell off the switch thingy.
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Eloquently put but perhaps you have 
do you have any semi-auto / full auto selection at all?
As well as Maiphut’s suggestion about the shot counter, check your JST connector pins in your battery plugs too. It’s possible you’re re-establishing a loose connection each time you plug in then working loose as you fire.
Yea I had trouble single trigger press fired a 4-5 shot burst. Wasn’t the fet ended up wire desoldered itself from the forward shot counter. Makes me think that’s the problem as that controls semi, burst etc.
But I could be wrong the fets do fk up on 3s alot.
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What does the shot counter look like .
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I’ve never had that problem with any of my AUGs but good information in case mine decides to play up… cheers! 
Definitely something to keep in mind if you’re like me and split receiver shells for paint 
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Black micro switch thing, front left of gearbox where two wires go from mosfet to the switch.
Cheers thank you, the black Micro switch .
Awesome pick up and advice
This was always 98% of the issues with the very first blasters ever released with select fire MOSFET equipped features on the SKD M4SS/HK 416 blasters.
It was the stupid micro switch that counted the number of piston compressions relayed back to the MOSFET.
They mounted that little micro switch right in line with the main spring/spring retainer and had quite a lengthy actuator arm attached to it.
Any slight movement in the spring or retainer would dislodge the metal arm from the switch and either create the same problem here…… or as experienced a few times, the metal arm would drop down into your gears and cause a bloody catastrophic failure to the gearbox 
I used to trim down the metal arm’s and add extra meat in the form of epoxy to keep everything braced up and in place for decent reliability.
I even got to the point of buying a heap of parts to utilise either an optical or magnetic shot counter switch…… but my bush mechanic modifications worked well enough that I never followed through with the more advanced upgrades.
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Hi , checked the wiring seems ok , symptoms , plug in battery, select single shot 123 shots , it stops , pull trigger, nil firing , nil 3 shot or auto, unplug battery, replug , again select 3 shot burst , fires x 3 , then stops , again unplug , replug , then works .??? Mosfet problem???, I didn’t realise how fragile they can be , or temperamental, I’ve 2 LH MOSFETS arriving soon .
If you checked the shot counter and it’s fragile wires running outside the gearbox , then yes it could also be the mosfet fried.
Be one of the two. Shot counter together with the fire selector tells the fet when to power the motor and make it go.
Just make sure that shot counter is functioning, reading off the tappet plate (white thing under nozzle) then move on to the next chain in the puzzle. It’s all good and half the fun. I’ll try help as much I can no problem
Like I said mine was a simple wire disconnection at the switch, chinese soldering at it’s finest.
It will do 1 press of the trigger regardless of selector fire 4-5 shots then shut down until you disconnect and reconnect battery. This can also be the mosfet. But while you wait for that check the rest out, change the main spring to a 1.18mm Ausgel. Uses all V2 internals tappet spring can get bit stuck.
If you do not have free tappet plate movement could be a problem. SHS V2/V3 tappet plates fit but if you overtighten the acryllic gearbox it will seize the tappet plate up which overheats the mosfet which can fry that too.
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who’s the real manufacturer of this?
Waiting for the mosfet , darn postage , my other project hybrid 92 gear box for my acr, problems , auto , no semi . Shs bits , 16 /1 gears , select arm Shs , butttttt, , any tips
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Don’t buy from them they are pricks. Challenged me to a punch up with his druggo mates. Fukwits.