I originally wrote this in reply to someone in the Gen.Tech area. More & more people asking about similar problems, so I copied it here. This should apply to Accord 2003-2006 with manual heat (not auto-climate-control). If anyone wants to add something about the ACC system, have at it.

First, check to see that the WHT/BLU wire at the blower motor is getting battery voltage when the key is ON.

Under-hood fuse box:
fuse #22 (100A)
fuse #21 (40A)
fuse #23 (50A)
Under-dash fuse #30 (7.5A)

Under-hood fuse box; find the blower motor relay. As you stand in front of the car it's back row just left of center. Blades are arranged like this:
- (pin #1)
- (pin #2)
| | (pins 3 & 4)

Top 2 pins (1&2) they should have continuity between them when you apply battery voltage across the other 2 pins (3&4). You can swap this relay with others having the same configuration of pins (for example the power-socket relay).

The power transistor is on the ground side of the blower circuit, so do the above checks first.

Unclip the wire from the blower power transistor. For some strange reason they're different for 2-door & 4-door cars.

2-door - measure resistance between the 2 wires closest to the lock/latch of the plug.
4-door - measure resistance between the 2 wires farthest from the latch.
Should be between 1.4k - 1.5k ohms.

2nd part of the test... Remove the #1 wire (4-door) {#4 wire on a 2-door} from the plug (should be BLU/YEL). Insulate it from ground & then re-connect the plug without that wire. Supply battery voltage into the empty socket where that blu/yel wire came from. Key ON, the blower should run.

One more thing...
The black wire that goes to the power transistor is ground. Measure resistance from that wire to a good body ground.

 

For the blower motor itself...
WHT/BLU is power. Should always be energized when key is ON.
BLU/BLK goes to the power transistor for speed control.

For the blower power transistor...

BLU/BLK is the wire coming directly from the blower motor. It'll be energized when unplugged & key ON. Touch it to ground (only for a moment) to check the motor runs (full speed).

BLU/RED & BLU/YEL both go to a solid-state circuit within the audio/HVAC control panel. That must be driven somehow by the blower-speed control ****.

BLK is ground.

 

Thanks to this forum, I was able to troubleshoot and repair the A/C system in my 2004 Accord. Earlier this week, the A/C system stopped blowing air altogether and I immediately thought the worst but hoped it was a fuse. After checking the fuses and swapping relays around under the hood, I hit the Internet and found this forum where I found a discussion on the Blower Motor Resistor. It took some looking to find it, but I have a crappy Hanes manual for my wife's 2003 CR-V that lead me to the firewall on the passenger side of the car. In case it might help someone else, here's a bit of a description:

1. Under the glove box, there's a flexible panel that needs to be removed; just push/pull at one side and it should pop right out. That will reveal the blower motor directly underneath.

2. You'll see the Blower Motor Transistor (that's what the replacement part is called) mounted on the firewall. It's left of the blower motor and close to the center console, and has a 4-wire connector block with a white plastic connector plugged into it. Two phillips head screws hold it in place.

3. Getting the connector off takes a bit of effort. Squeeze down on the tab on top and work it back out. To remove the part, I used a relatively small screwdriver to remove the left-most screw as access to that screw is somewhat obstructed. The part is easily removed once the screws are out.

4. Pop the new part it and replace the connector! Viola! Air Conditioner is back in business! Of course, I plugged the new part in and let it "dangle" first to see if it was the problem.

The part was $65.68 from the Honda dealership ... cheaper than any AutoZone, DAP, etc., quote I got. That said, I'm sure Honda would have charged $200-$300 to "fix" the problem. The same dealership wanted to charge me $425 to replace an O2 sensor on the CR-V ... yea ... literally a 5 minute job and a $140 part (Denso OEM drop in).

I was able to bypass the part to determine that the blower motor was good prior to buying the replacement. I used a clip lead to the car frame to ground the blue/black wire on the blower motor. I had the key in the ignition and engaged all the way without starting the car as I felt this might be required to energize the blower motor relay which provides 12V to the blue/white wire on the blower motor.

Anyway, I hope this description helps the next person that has this problem. In the end, a very easy fix so you can save a good bit of money over having the dealership do the work.

 

Since my post assumes you've ALREADY found the blower speed-controller, I copied the post from Vortecks into here.

 

Worked like a charm - THANK YOU.

The guys at O'Reilly Auto Parts had a hard time finding the part in their database, but once they did, it was only $59. That was honestly the hardest part of the install - getting the guys to understand which part I needed. I had stumped two of the guys, then finally a more knowledgeable clerk found it in the system as a "Blower Motor Resistor" and ordered it in for me next-day.

Whatever you do, don't throw out the terms "AC Power Transistor" or you will have them scratching their heads and looking for relays, etc.

 

I checked every fuse first and swapped black Omron relays with each other... no go..

I didn't quite understand how to find the Blower Motor Transistor. Are you saying it's within the two relay holding device? I will try and add photos.

We had just bought 3 weeks ago, a Honda Accord 2003 EX from a dealer AS IS. Only has 75K miles.. and had a Honda dealer check it first.
Everything good right? WRONG

I have the yellow engine light on and
The AC blower doesn't blow anymore for no reason at all here..(Texas 103deg.) ;-(

Also, trying to figure out how to do a self-diagnostics..

[quote=Vortecks;222125]Thanks to this forum, I was able to troubleshoot and repair the A/C system in my 2004 Accord. Earlier this week, the A/C system stopped blowing air altogether and I immediately thought the worst but hoped it was a fuse. After checking the fuses and swapping relays around under the hood, I hit the Internet and found this forum where I found a discussion on the Blower Motor Resistor. It took some looking to find it, but I have a crappy Hanes manual for my wife's 2003 CR-V that lead me to the firewall on the passenger side of the car. In case it might help someone else, here's a bit of a description:

1. Under the glove box, there's a flexible panel that needs to be removed; just push/pull at one side and it should pop right out. That will reveal the blower motor directly underneath.

2. You'll see the Blower Motor Transistor (that's what the replacement part is called) mounted on the firewall. It's left of the blower motor and close to the center console, and has a 4-wire connector block with a white plastic connector plugged into it. Two phillips head screws hold it in place.

3. Getting the connector off takes a bit of effort. Squeeze down on the tab on top and work it back out. To remove the part, I used a relatively small screwdriver to remove the left-most screw as access to that screw is somewhat obstructed. The part is easily removed once the screws are out.

4. Pop the new part it and replace the connector! Viola! Air Conditioner is back in business! Of course, I plugged the new part in and let it "dangle" first to see if it was the problem.

/quote]

 

This was and is a very good post..remember where it's at members..Thank You Jim Blake.