It was unusually cold last night when driving home late (mid-30s, cold for California!), so I cranked the heat up high (max) for the first time in ages ("Hi" setting). With the 'auto' setting, asking for heat puts the dampers in the 'foot' position, and warm air could be felt at my feet - good. Also, warm air could be felt at the dash side vent - good. But a very slight amount of cold air could still be felt coming out of the main central dash vents; not a lot, but enough to be unpleasant. So I pressed the 'mode' button to switch from 'foot only' to 'foot' plus 'head' - down arrow plus level arrow, to cause some warm air to come out the central dash vents. This air was extremely lukewarm at best. The air from the left side vent was quite hot (and the air delivered to the rear seat area was hot too, so we know the coolant is hot). I then adjusted the mode button to switch the dampers to 'head' only - level arrow only, so that all air would be directed to the central dash vents. Even now, with the engine fully stable (hot), and after 30 minutes of driving, the air coming through these central dash vents was barely lukewarm. I adjusted the 'recirculate' button (outside/inside air) and it made little difference (just noisier / quieter). The temp settings was as high as it would go, and hot air is felt at all times on the dash left side vent. I turned off the A/C just in case that mattered, no difference.

I recall now experiencing this last year - big fight with the g/f asking me why I was blowing cold air when it was so cold outside!

I don't know where to begin troubleshooting this. An air lock in the coolant system? A damper issue? Thanks!

 

Could be an issue with the HVAC center stack's servo motor blend doors. The doors may be stuck in the "mostly closed" position.

 

Is that an expensive fix?

 

Depends upon whether or not you can do it yourself. You must carefully disassemble at least some of the center vent stack to get to the servo motor and blend doors assembly. Sometimes all they need is a good cleaning. Sometimes the motor has failed and needs to be replaced. Google "automotive HVAC servo motor and blend door repairs" and read up on it. Every make and model is slightly different, and some are more prone to these issues than others.