Hello,
I have an 01 Civic EX. A/c blew very cold. On a hot day ran into a store, came out to start the car and it blew hot air. Checked the fuses and all seemed fine, but just hot air.
Took it to two mechaniscs, both said the compressor was fine and coolant was fine with no leaks. One mechanic told me it is "probably an electrical realy issue", but could not tell me how much it would cost untill he took the car apart. I was not comfortable with an open ended mechanics bill.
Can you please let me know of some DIY troubleshooting that could help me isolate the real issue here. Any help would be greatly appreciated. Thanks!

 

So when you press the A/C button, the compressor does not engage?
What does happen when you press the button? Does the light in the button come on? Does the engine idle speed change? Do the fans under the hood come on?

 

When I press the A/C button, the light comes on and I can hear the engine idle speed increase slightly, but just blows out air slightly hotter than the outside temperature.

 

also, as far as the fans under the hood, I am unsure what I should be looking for, but all seems to be normal

 

Both the radiator fan and condenser fan should run when the A/C button is pushed. If so, you may want to test the thermal protector on the compressor and the compressor clutch gap.

 

I think I am good enough mechanically to handle replacing the thermal protector. The compressor clutch test may be a little over my head. How can I test the thermal protector?

 

Test the unplugged protector for lack of continuity across its two terminals. If it's open, replace.

The armature plate gap can be measured with a feeler gauge and a willingness to work in a tight space.

 

I'm guessing the OP's mechanic jumped the compressor and found the clutch was OK.

I think that on that generation, the thermal protector interrupts the signal before the ECU, so if it were open there'd be no idle increase and no condenser fans.

So that really just leaves the compressor relay. Open the underhood fuse box and swap the relay with one of the other ones.

 

Based on the circuit diagram, it appears that the idle would increase and the fans would run if the thermal protector or clutch gap were the problem.

Nonetheless, checking the compressor clutch relay is still a good idea. The fan and compressor relays can be swapped to test this idea.

 

Thanks for pulling the diagram, Ron. Diagram shows the old type with thermal protector in series with the clutch, and only one wire connected to the compressor. But the manual posted shows the newer type with a separate thermal protector with it's own plug. I think with those the thermal protector is wired in series with the pressure switch in the low-level control part of the circuit. OP will need to examine the car to see which type is actually in use.