Just had a MBPR 13” 2015 come in for repair with the same issue. After trying a replacement fan with no success I looked under the clamp latch and noticed a small amount of corrosion. After cleaning that off I reinserted the ribbon cable from the fan and noticed the clamp latch was not completely inserted on the connector properly. After closing the latch on the ribbon cable I put a small amount of pressure on the latch to push it into the connector all the way and when I turned the computer back on I was now getting an rpm reading and the Apple Hardware Test was no longer giving an error code. Hope this helps!