Replaced fan - worked until MBP wanted to use it. What happened?

Lets start off trying a better app to monitor your systems thermal sensors and fan. While iStat can give some useful info some versions have problems (the free one). Instead lets get this better tool: TG Pro. For less than $20 US it's the best tool out there. Do make sure you remove iStat first.

Can you also take a good pic of the fan connection to the logic board so we can see it.

I'm not sure whats up here, you have me scratching my head ?!?!

The MOT4 error with a zero at the end is the fan so AHT is telling us its bad and it is reporting no RPM's just like TG Pro is. But, the diagnostic function within TG Pro is telling us the fan is reporting back good. So do we have something messing it up within the system, is iStat still active? Or something over-riding the fan like Fan Cnt. You could try an external boot drive which has not been altered to double check.

Lets start off by doing a SMC reset. Follow this Apple T/N: Reset the System Management Controller (SMC) on your Mac make sure you push the system a bit so the system would need to turn on the fan, watching TG Pro did that alter the fans RPM value? If its still zero I think we need a new fan. You could also see if that clears the AHT error.

So the question here is: Is the replacement fan defective or is there something more going on within the logic board. Looking at your images I don't see anything visually. At this point I think we have to try another fan unit.

Post or call the IFIXIT Store (contact info) if you bought it from them our sponsor as you don't want to wait too long for a swap-out return. Post a link to this dialog for them to see whats transpired.

Let us know what happens. Good Luck!