After owning the truck for more than 5 years, the fan speed in AUTO mode on my 2011 Lariat Limited (dash picture below) always settled down to a more moderate setting after the cabin had cooled off (or heated up in the winter). And I can usually manually override the fan speed by adjusting the temperature by one degree. This summer is the first time I've noticed it sometimes staying in the highest fan speed indefinitely, and it only seems to do it maybe 15% of the time. I can confirm it's stuck by adjusting the temperature by several degrees (which would normally make the fan speed slower). I usually notice it's happening once I'm 10 minutes down the road and my A/C is still blasting at maximum airflow. I have no other symptoms, noises, or problems with my A/C performance--everything works great except for the sticky fan speed in "Auto" mode. And when the system selects a slower speed, the other fan speeds do work fine (which seems inconsistent with what other people reported when their blower motor resistor went out). It seems that the only way to override the max fan speed is to power off the HVAC momentarily and then turn it back on to reboot the system. This usually restores proper function and the airflow resumes at a more appropriate medium setting (that is much quieter too).

Based on a quick search of the archives, my research suggests this may be the beginning of a blower motor resistor failure, but it seems most of those people had a different style HVAC control panel (with round dials to adjust fan speed?) and the automatic systems are "pulse width modulated." Am I barking up the wrong tree?

 

DATC-equipped vehicles don't use resistor packs, they use a blower speed control module.

It's either the BSC module or the DATC module.

 

Thanks. Are those easy to test and replace?