1999 Honda Accord can't change ventilation mode

It's a vacuum line. Rubber tubes run behind the firewall to control the vents. A rubber tube has likely deteriorated over the years.

You can look for the tubing under the hood, coming out of the firewall. Oftentimes, a tube will disintegrate near the hot parts of the engine where it obtains its vacuum source. If not, you will have to remove plastic cowling over the firewall and trace the tubing deeper into the dashboard.

I'm not 100% positive on this, but I think the blower door actuators are electric on the accord, not vacuum. To test this, I sat in my 01 Accord (same car different year). With the key in the accesory position (no vacuum), I can switch between all of the different blower configurations with no problems. You can even hear the little doors moving around in there.

So get in it, put the car in accesory mode, and try to switch between the various buttons. If you don't hear anything, my guess would be the vent door actuator motor is toast, or whatever fuse it is on is toast. Not sure what fuse that is exactly, but you can look at the fuse diagram in the owners manual.

From what I've found online, it doesn't look like you need to pull the whole dash apart. The part under the center control console has a few screws holding in the trim that allow you to get at these elements. Hopefully.

Put the HVAC unit into diagnostic mode and it'll tell you what you should be looking for.

Key On Engine Off, turn off the blower motor, place the temp control knob all the way to full A/C the recirc light will come on. Press and hold the recirc button and the light will turn off. Keep holding the recirc button until it starts flashing.

1 blink is for the air mix motor.
2 blinks are for the mode control motor.
3 blinks for the evap temp sensor.

On this year Accord the mode door is cable controlled by an electric motor. Take a look under the left side of the dash and you should see a 7 pin connector for the mode control motor.

Hit a button, does the motor move? if it doesn't disconnect the motor and apply power and ground to pins 1 and 7 this should place the motor into the defrost position. If it still doesn't move replace the motor. If the motor does move you'll need to remove the control panel and check for continuity between the panel and mode motor. if the wiring and motor check out replace the control panel.

I had this happen to me right after I replaced some engine mounts. The power to the HVAC control panel would be intermittent.

I tried changing the panel but no luck. Still intermittent power.

I traced it to my bonding straps and battery ground cable. I replaced a couple of the engine bonding straps and cleaned all the the connectors on the ground cable and it works fine. Give that a shot. Lights came on right away and was able to switch functions again.

Luke

The mode control motor is easily visible from the driver's foot area. It's on the side of the center console. Change the setting on the control panel to bi-level (Def/Foot) and wiggle the control rods until they move to the bi-level position. This will hold you until you receive your new MOTOR ASSEMBLY, MODE- 79140-S84-A01 which you can get from any Honda parts source, in person or online.