Starting issue
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
Again, all I have got for a scan tool is HPTuners, nothing “factory level”. The new ECM is brand new from GM and was flashed at a Chevy dealership.
I may be misunderstanding your request, you say check terminal 85 at the ECM and to back probe at the ECM. How would I go about doing that? I want to make sure I am doing it correctly.
When I go back out there, I will share a screenshot of what HPtuners is telling me.
And woah, guys. Let’s just relax here, you’re both being helpful to someone who knows not much but more now
you guys are doing good work as far as I can tell. Let’s try to refocus and be cool and respectful to one another
I may be misunderstanding your request, you say check terminal 85 at the ECM and to back probe at the ECM. How would I go about doing that? I want to make sure I am doing it correctly.
When I go back out there, I will share a screenshot of what HPtuners is telling me.
And woah, guys. Let’s just relax here, you’re both being helpful to someone who knows not much but more now
you guys are doing good work as far as I can tell. Let’s try to refocus and be cool and respectful to one anotherLast edited by Gill; Apr 28, 2026 at 06:22 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,546
Likes: 4,021
From: Cape Coral, Florida
You need to back probe pin 72 with ignition on or and check that it may not be a broken wire between the ECM and terminal 85 of the relay...understand what we are trying to check here ??...if jumping the load side of the relay (30 and 87) or applying 12 volts to 87 can you establish communication with the ECM ??
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
Yea, one thing that I wanted to test tonight is can the ECU communicate if I jump KR75’s load terminals. Before I test that though, I will confirm if X1-72 lights up when the ignition is on. Additionally, I will take some pictures of what HPTuners is telling me for DTCs. Are there any specific PIDs I should add to the scanner?
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
Hooked the ECM back up with battery unplugged and connect to pin 6 and pin 14 of my OBD plug with my multimeter on the 200 ohm setting. It read about 128-130 ohms. This seems high from earlier conversation… I was a fool and did not hook up my ECM the first test and it read about the same. After hooking up the 3 connectors, it still read 130-ish
Last edited by Gill; Apr 28, 2026 at 06:48 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,546
Likes: 4,021
From: Cape Coral, Florida
If you don't have a back probe you won't be able to check what we are trying to accomplish...HP Tuners is just a generic OBD2 scan tool...I don't think it will show so some of the data PIDS we are trying to see...I'd save your money for a good aftermarket factory level scan tool like Autel or Launch...I'd stay away from ones by Innova or Foxwell...Zurich too.
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
I still have my original ECM, so I threw that thing into the car and connected the connectors and now the resistance between ODB2 pin 6 and pin 14 is about 70 ohms. Not the 60 mark, but closer…
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,546
Likes: 4,021
From: Cape Coral, Florida
70 ohms is close enough...maybe your ohm meter...60 ohms is telling us the wiring is good...130 is not good so looks like the ECM !!..2 part video if you want to learn some of this stuff.
Last edited by C5 Diag; Apr 28, 2026 at 07:52 PM.
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
So, as stated in my previous post, my new ECM was reading 130 ohms over OBD2 pin 6 and pin 14, I decided to swap my old ECM for a control group test. When testing it, I saw that it was reading about 70 ohms, so I said to hell with it and hooked up the battery and HPTuners and voila, the the old ECM was talking. So, I disconnected the battery and jimmied the connectors again and re-tested pin 6 and 14 which then read 61.8 ohms. I have yet to try to start the car since the battery has gotten low (11.5v) due to me leaving it on during my testing. I am going to pull the battery and charge it up and report back with what I discover.
For more context, the locking tabs on my X1 and X3 plugs on the old ECM (not wiring harness) were damaged by my own hand, and do not provide a good locked seat to the ECM. I had previously swapped the old ECM back in when the NO COMM issue started, but for whatever reason, the old computer would not talk either (I may have failed to seat the all the way on the connectors, I really don't know).
For more context, the locking tabs on my X1 and X3 plugs on the old ECM (not wiring harness) were damaged by my own hand, and do not provide a good locked seat to the ECM. I had previously swapped the old ECM back in when the NO COMM issue started, but for whatever reason, the old computer would not talk either (I may have failed to seat the all the way on the connectors, I really don't know).
Last edited by Gill; Apr 28, 2026 at 08:10 PM.
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Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
The old ECM has no issues with it aside from the locking tabs being broken. All of my issues started once the new ECM was put in. As I stated, once the NO COMM U100 error was popping up on the new ECM, I put the old ECM in and it did not talk either. Why? I could not say. But both computers failing to talk led me down the path you assisted me on. Regardless, I have a feeling the car will start just fine with the old ECM once the battery is charged up, but that is yet to be seen
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,546
Likes: 4,021
From: Cape Coral, Florida
The old ECM has no issues with it aside from the locking tabs being broken. All of my issues started once the new ECM was put in. As I stated, once the NO COMM U100 error was popping up on the new ECM, I put the old ECM in and it did not talk either. Why? I could not say. But both computers failing to talk led me down the path you assisted me on. Regardless, I have a feeling the car will start just fine with the old ECM once the battery is charged up, but that is yet to be seen
Lets hope for the best !!
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
3rd Gear
Joined: Mar 2026
Posts: 3
Likes: 0
So, as stated in my previous post, my new ECM was reading 130 ohms over OBD2 pin 6 and pin 14, I decided to swap my old ECM for a control group test. When testing it, I saw that it was reading about 70 ohms, so I said to hell with it and hooked up the battery and HPTuners and voila, the the old ECM was talking. So, I disconnected the battery and jimmied the connectors again and re-tested pin 6 and 14 which then read 61.8 ohms. I have yet to try to start the car since the battery has gotten low (11.5v) due to me leaving it on during my testing. I am going to pull the battery and charge it up and report back with what I discover.
For more context, the locking tabs on my X1 and X3 plugs on the old ECM (not wiring harness) were damaged by my own hand, and do not provide a good locked seat Zulekha Haywood to the ECM. I had previously swapped the old ECM back in when the NO COMM issue started, but for whatever reason, the old computer would not talk either (I may have failed to seat the all the way on the connectors, I really don't know).
For more context, the locking tabs on my X1 and X3 plugs on the old ECM (not wiring harness) were damaged by my own hand, and do not provide a good locked seat Zulekha Haywood to the ECM. I had previously swapped the old ECM back in when the NO COMM issue started, but for whatever reason, the old computer would not talk either (I may have failed to seat the all the way on the connectors, I really don't know).
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 36
Likes: 1
From: DFW, TX
I am uncertain what you’re asking, but if you want to know how to pull the wiring harness X1, 2, and 3 from the ECM, I can help! It is a snug job, but you are able to pull the red tabs on the cantilever to unlock them. Once those are unlocked, on the top of the connector there is like a little finger grove, you press than inward toward the ECM and pull the cantilever up and over. That’s about it… let me know if you need any other help. This is a great video showcasing the E92 ECM that’s found in C7.
https://youtu.be/PY7qNIFwE28?si=OUxVJ1G3op-355ps
https://youtu.be/PY7qNIFwE28?si=OUxVJ1G3op-355ps