My daughter's 2004 Lincoln V6 LS, Climate Control with Heated Seats
Did this repair this week. I found the information here to be disjointed so I thought I would summarize my work for the next person who is frustrated by their LS's HVAC system.
Obtain the trouble shooting codes from the HVAC unit.
PRESS "OFF" AND "DEF" followed by "AUTO"
My resultant code was 1947 indicating an Evaporator Discharge Temperature Sensor failure. There is one on either side of the HVAC plenum which are accessible while laying on your back looking upward under the dash. I am 6'2" and 240 LBS. No fun for a bigger guy!
I guessed at the drivers side as it was behaving the most erratically. I guessed correctly.
Swapped out the sensor. HVAC system works great and error code verified to be gone.
Only the rectangular portion of the sensor is visible. There is a 2" narrow protrusion that extends into the plenum. Be gentle removing it. Pop it loose with the blade of a medium size screw driver and gently rock it back and fourth to remove.
I paid $48.00 Canadian for the replacement sensor at APC.
In addition, the cabin ambient temperature sensor is located above the ignition switch in the dash behind a integrated grill. After reading here that it was a good idea to clean it, and an inspection revealed a lot of accumulated dust, I carefully blew it off with compressed air. I would presume a clean temperature sensor would help ensure the system performs at optimum.
Good luck with your LS. Fussy cars for sure!