Passenger side blows heat when AC is on?

That would typically be due to a bad right side air mix controller (blend door). It is behind the glove box door area on the HVAC box. It could be stuck in the hot position if it is defective. To access it you remove the lower trim panel and the glove box from the dash. Then you remove the duct from the side of the HVAC case and finally the actuator. Note the pin and lever circled in the image, that lever may have broken or the actuator pin failed or the actuator itself went bad. You can remove the glove box and duct then watch to see if the lever moves, no motion is usually a bad actuator. I would advise using an OE part
if at all possible, I've gone to salvage yards to get OE parts when they were NLA or on global back-order. Honda 79170TK8A41 Most of the aftermarket ones don't last all that long, I've had them fail in under a week!

Really the blend door would make it hot air? This isn't warm air like from outside, but very hot like the heat is on.

Yes. The blend door controls the temperature of the air. Air enters the HVAC box, either from outside or from inside depending on what the recirculate controls is set on. From there the air goes through the filter and into the fan, the fan blows it into the main AC case where it then goes through the AC evaporator core first. Then it hits the blend door. The blend door controls the airflow, if you select AC then it stays in one position and directs the cold air out of the case and into the vehicle. If you select any type of heat though the blend door opens and sends heat into the heater core area of the case. That gives you anything from mildly warm to full hot depending on how far the door moves to direct the airflow through the heater core. The air then hits the mode door(s) those direct the air through the defrost, vent or floor ducts depending on what is selected.
Your vehicle has dual blend doors and 2 actuators. The air through the case doesn't actually split until the blend door so if one is stuck on Heat and the other to AC you get the issue you have. One side hot from full heat through the core while the other side tries to cool it. It's also the reason I think that the dual zone (or more) systems are a waste of money and effort in a vehicle. Now if there were dividers to keep the zones separated it might make sense. Otherwise you get the result you get as well, the hot side just causes the entire vehicle to stay warm.

Ok I ordered the new blend door and cabin filter, thanks

Let us know what you find. The actuator and lever are the common failure items. If you have an inspection camera you could check it without taking things apart.

Ok I replaced the passenger blend door actuator and now it's all working as expected. The old one barely moved anymore. Also replaced the cabin air filter while I was in there.

Side note, I checked the temps in all the vents and got some strange results. The passenger is reading 50F, and the rear is 54F. But the driver is reading 66F. Could the driver blend door be stuck partially between or is something else going on?

Could be that the seals are getting bad on the doors or the door isn't closing fully or it might be that it learned a slightly open position. A reset using a scan tool might take care of that but it may not if the issue is mechanical.

I watched a video of how to replace the driver side blend door, it looks way easier than the passenger side. I will try to watch it and see if it's moving fully and decide. Thanks

If it is moving OK a reset might fix it rather than replacement. The system learns the distance the actuator moves from end to end. However it can get corrupted over time.

Yeah you mentioned a code scanner can do it? Or can I just unhook the battery to do it?

It depends on the scan tool you have. It has to be one that can do bi-directional control of the HVAC system. You can try powering it down by pulling the HVAC fuse 16 in the under hood fuse box at the right rear of the engine bay BUT be sure you have the unlock code for the stereo as it could cause it to lock up.

Ok I pulled the fuse 15 and 16 (16 was labelled "backup" and 15 was "radio") and that did make the actuators reset. But it sounds like the driver side can't train it just keeps making noise like it's going nutz. I can't really see the arm moving on the actuator due to there being a hose in the way, but I may go ahead and replace it since it's so easy to get to and cheaper than the passenger side for some reason...

PK, that sounds like the potentiometer in it has a bad spot, that will cause it to stop in the wrong spot. The reset revealed that issue because the calibration uses two item, one is the pot. the other is the motor current. When the door closes the motor in the actuator generates a current spike because it wants to keep moving, the control module sees that spike and reads the potentiometers position and says "That's one end", then it runs the motor the opposite way until it stops, and locks that in as the other end of travel. If the pot is faulty it will try to keep running like yours is. The left side is easier to get at and they sell more of them because the base version only uses that side not both so they are a little cheaper. When you install the replacement move the arm to be sure the door moves properly, then put the pin in the slot and put in the screws. Then connect it and reset and see if it all works again.

Ok thanks again the new one is on the way, it was 1/3 the cost of the other one!

One last thing, is 50-55F ok temp at the vent for AC? That's at idle, not driving down the road (I haven't had a chance to check it while driving).

Check it with the AC on full and after it runs a bit so the ductwork has cooled down. Then look for a 20 degree drop over ambient air temperature as "normal" operation. So if the external air is 80, 60 in the interior is "normal" In the vent after a cool down that could be as low as 40 on a 60 degree day up to 70 on a 90 degree day.

Ok it was around 80F outside on the day I replaced the other blend door actuator and checked the temps, so I guess 50-55F is within spec

Thanks for all the help

Yep, should be OK.

Ok driver side blend door actuator swapped out and now it's blowing 50F on LO like the passenger. Thanks again

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