2003 Supercrew Lariat with electronic temp control module. I keep it on Vent mode 90% of the time. Noticed strange operation a couple times this winter, now seems to do it all the time. Truck up to full temp with Vent on.....I turn EATC up to 90 degr.....but air coming out of vent is luke warm. Switch to FLOOR.....same temp out of floor. Switch to Panel/Floor setting....suddenly temp increases to full hot. Switch back to VENT or FLOOR....temp immediately goes back to luke warm. The only way I get full temp with EATC set to 90 degr is in the Panel/Floor mode or Automatic.

Also, with EATC set to VENT and 90 degr, luke warm air coming out.....if I shut off engine with those settings and restart, Vent will go to full hot like it's supposed to. Also works on FLOOR setting as it should after the restart.

First thought I had was blend door...but why would it work fine in the Panel/Floor or Automatic mode? Could this be a vacuum issue in EATC?.....but blend door is electric motor driven. Once I restart, the EATC seems to function just fine in VENT if I lower the setting down to 60 degr then back up to 90.

 

http://www.idmsvcs.com/2vmod/eatcswap/selftest.html

 

Thanks.....ran that self test.....no codes.