Heater Stuck on Defrost

Hi All,
I have a 2004 Tahoe z71 w a 5.3. Just had the engine replaced (long story, bad heads). Happy with the new engine but my heater is now stuck on defrost. Can't get it back in the shop until next week, so want to see if I can diagnose. I want to check the vacuum lines but don't know where to look. Can someone please point me in the right direction on where to look?

Thanks much!

Hi All,
I have a 2004 Tahoe z71 w a 5.3. Just had the engine replaced (long story, bad heads). Happy with the new engine but my heater is now stuck on defrost. Can't get it back in the shop until next week, so want to see if I can diagnose. I want to check the vacuum lines but don't know where to look. Can someone please point me in the right direction on where to look?

Thanks much!


disconnect the battery negative connection for a few minutes , turn key to on to discharge system ..then reconnect the battery...


this may reset the system..

Thank you for the idea about the reset. Unfortunately, it did not work. Any other suggestions please?

It all worked ok before the engine repairs? If so, Jcat may be on the right track. What I have learned is to disconnect both battery cables, then hold them both together with the ends touching for several seconds. This will discarge all capacitors. Then reconnect the battey. Hope this helps!

Hi All,
I have a 2004 Tahoe z71 w a 5.3. Just had the engine replaced (long story, bad heads). Happy with the new engine but my heater is now stuck on defrost. Can't get it back in the shop until next week, so want to see if I can diagnose. I want to check the vacuum lines but don't know where to look. Can someone please point me in the right direction on where to look?

Thanks much!

you need to give what ac control system you have . manual or auto ?

try the reset as mentioned again. it is possible the computer controller got a voltage spike.