2002 Honda Insight, the air is not working. System is charged properly and not leaking. Here's what the shop (small Indy) said word for word:
On the Insight. The computer is not sending ground to the a/c relay. The ecm is not communicating with either of our scanners so we can not get in and see what's happening on that side of things. We also checked all fuses on the car and everything is good. Our system is not giving us enough info to go any further checking it out.
Likely the ECM is bad? Car runs fine otherwise. Is the ECM easy to swap if I find a salvage?
I wonder what the high side pressure switch is doing (or not doing).
If the shop cannot communicate with the ECM I would assume the ECM is actually fine until proven otherwise.
I would probably jumper the high side pressure switch while the A/C is turned on and the engine is running to see if the clutch cycles. NOTE: I have never touched one of these cars.
Highly unlikely that a module would fail to communicate but still work otherwise. Can they communicate with anything else, or is something else pulling the network down? That's what I'd want to see first.
I mean, it is still a possibility that it's a fried module, but that's a pretty expensive guess and a lot of "I just replaced the computer and it still doesn't communicate" turn out to be because of something much cheaper to fix.
An aside, most shops have five or six different scan tools, because nobody has 100% coverage, or maybe THIS scan tool is good at graphing but THAT scan tool is the only one good for reprogramming Honda keys, or whathaveyou. Yeah, it's expensive. That's part of why diag time is $180 an hour. Hell, I personally have one handheld and several different laptop based ones, and most techs I know have their own scantools to supplement shop supplied ones, and everybody borrows everyone else's on an as needed basis. (Or more like, "hey I see you're having problems, try using MINE, because it rules" 
)
Its pretty basic, so I doubt they did not check, but I have seen a pin on the OBD2 plug pushed in and not making contact.
Could this be the issue their scanner was not able to communicate?
In reply to Slippery :
I'm not them, but any time I can't communicate with a module, I try to see if I can communicate with anything else. Then the next step is verifying power, ground, and network signals at the DLC if nothing is talking. Don't even need to know if this CAN line is 2.5-5v or 0-2.5v or 0-5v or anything like that: just looking for railed to ground, railed to 12v, looks funny, or whathaveyou. This can be done with a simple DVOM, but a scope will let you see, if there IS a valid CAN voltage, if the only data passing through is the same short burst over and over again, which is usually machine speak for "Hello? Hello? Hello?".
All of this gets documented. That way, if the problem recurs, the next guy (might even be me) can go back and see, okay it was doing THIS four months ago. And the info can also be passed along to the car's owner, who may not know or care about it, but maybe their uncle or neighbor or someone could, and if there is anything technicians like to do, it is pick apart other guys' diagnoses 
I am at a slight disadvantage because I no longer have home access to service information like I used to, but assuming that the Honda uses high speed CAN for communication between the engine controller/hybrid controller/ABS/airbag systems, if you are having comms issues with the engine controller, you either have a problem at the DLC, a problem with your scan tool, or a complete Christmas tree on the dash. Generally.
I have found with my wife's prius that even a good scan tool has issues with hybrid related faults. Being a hybrid does it have a high voltage electric a/c compressor or does it still use a engine driven compressor?
With out the wiring diagram it's hard to say how the compressor is controlled but I would think a ecm failure would cause more issues, their communication problem could just be the scan tool used.
Caveat- I prb have .001% of Pete's knowledge, but I have been able to successfully troubleshoot Honda AC with just a test light (98 & 2011 Civic's).
My anecdote-based guesses would be relay, the temp sensor thing, or the clutch has worn enough that it won't engage anymore (sometimes just removing a shim can bring it back in tolerance)
I think the most critical thing that the shop told you is that they don't have the tools to troubleshoot the problem. The lack of command from the ECM could be an an ECM problem but it's more likely that the ECM is not seeing the right combination of inputs to send the signal. The fact that their scan tool can't talk to it doesn't mean a thing unless they know that it usually can talk to 2002 Insights.
Well, you could momentarily ground that terminal on the A/C relay to see if the compressor clutch activates. If so, and the A/C works fine, then you know it's a problem with the ECU. Pressure switch in the refrigerant line will not allow the clutch to activate if the pressure is too low (no Freon and/or lubricant), or the pressure is too high. Those switches do go bad.
In reply to earlybroncoguy1 :
Or there is a wiring fault.
Or a fault in the relay box (I forget what Honda calls it)
If any of it has to pass through the MICU, that is always suspect. I am pretty sure that it's not involved this time.
Need to remember to get in to work early tomorrow to check it out. I don't have my SI set up on the tablet that I take home, just my work laptop.
