Code Reader cannot communicate with ECU/PCM - Guide for stock ECU swaps
 

This was copied from a response to a member, but since this crops up from time to time, better have a summary

a loose alternator can cause it to fry the PCM.
This is a known common issue in these cars.
If ECU/PCM does not come back after fixing alternator, look for used here:

https://www.civicforums.com/forums/1...-part-com.html

Notes:
1) get from same transmission type. (some sensors in the auto trans vs manual and the ECU/PCM might not like it)

2) If you have an EX, get from an EX - EX have the VTEC, the others do not have. If ECU/PCM is looking for VTEC and it is not there, engine will go into limp mode.

3) Starting from the 7th gen, (2001-2005) there is an anti-theft function - key have a chip that the PCM will read - if not programmed, it will not start. Looks like regular keysmiths nowadays can program them too, otherwise you will need to take to a dealer if you can't find one to do it. Or you can see if you can get the PCM, keys and drums from the same car in the junkyard (mostly the ignition that detects the chip, but if you want only one key, need doors, ignition, trunk and the rear seat lock). PCM should be around 100 bucks, not sure about the keys/drums. They will need to come from same car

4) 01-03 have narrow band O2 sensors, 04-05 have the wideband O2 sensor, so recommend getting PCM from 04-05. Not sure if you would have issues with the wideband sensor coupled with narrow band PCM. Having a narrow band sensor coupled with a wideband PCM for sure will not work.

Re: Code Reader cannot communicate with ECU/PCM - Guide for stock ECU swaps
 

You cannot swap one type of sensor for another. The PCM cannot adapt.

Re: Code Reader cannot communicate with ECU/PCM - Guide for stock ECU swaps
 

...I can't find the rep button on the phone app. I'll try to rep this post when i get home. Good post.

Re: Code Reader cannot communicate with ECU/PCM - Guide for stock ECU swaps
 

i always have to 'view full site' instead of the mobile version

Re: Code Reader cannot communicate with ECU/PCM - Guide for stock ECU swaps
 

I will add to this discussion the matter of checking the three relays (blue, brown, black) in front of the ECM. I did a communication test between my replacement ECM and the OBD scanning port before fully installing it. At first I still saw the linking error of my OBD scanner not being able to talk to the ECM. I replaced the relays one at a time with units from the car I got the new ECM from. I swapped out the old and tested until the linking worked. I left the replacements in as I went. I ended up having to do all three. The black one was definitely involved (last one to be swapped), so I’m not sure if the others were also bad.

Relays swapped, I now had good linking between OBD and ECM. As a test with the new relays in place, I double checked my original ECM, which really was dead.

Moral of the story, be sure you have good communication before you fully install/button everything up.