U0100, and P0700 popping up

Thanks avguy, it was your tip that found the TCM code (P0722 in this case) that nailed the problem, without taking a risk of messing up both cars lol. With some luck I should have the orange car running this weekend.

I spoke too soon apparently. Three days and almost 100 miles on the new speed sensor, and the light is back and limp mode came on. However it is not as bad, rarely does it but still same codes popping up. Ordered a used TCM to try next. They list the ECM as an option but I would think that would show other symptoms. I may be dealing with multiple problems.

Update: Still getting same DTC's with new speed sensor, and even does it with a different 'known good' TCM. So ruled out the TCM. Even the U0100 'network communication error' pops up as I drive it. So frustrating. I have run out of things to replace, so it may be going to a shop next. I am just dumbfounded. &^$%#

I checked for 12v at the OSS connector pin, checked it at the TCM connector, wiggled the wires, etc. Solid 12v signal. So if it is harness/wiring related I can not duplicate the problem by shaking wires. I have followed the tis test procedure and still get the same result. I even thought about 'hot wiring' power to the connector but no sense in butchering the harness. I am totally at a loss on this.

Looking at the wiring diagram, that sensor and the ECM share a circuit, so that seems to be a smoking gun. Have you checked their common ground - G103? IIRC it's located at the starter mounting bolt.

To be honest I glossed right over checking grounds aside from the one at the fuse box near the battery. I will chase this circuit down and see what I find. Again - thanks for the tip avguy.

Update: Found G103, (down on the bottom starter bolt with a gang of other wires and the ground cable. ) Pulled them off, everything was clean, so I cleaned them with contact cleaner and scuffed them up a bit, and tightened. Seemed to have good ground where I checked with a light clipped to the positive post. So grounds seem to check out ok. Still getting same codes however. I saw a splice 201? On the diagram, but was not able to find the splice. Off to the dealer next week, I am still stumped.

Update: Passed on the dealer, took it back to the shop that remedied the air bag light. No matter how much I drank, could not psych myself up to go to a dealership lol So right now the verdict is the ECM, they did the same thing I have done multiple times (cleaned the connectors, and cleared the codes) and the classic story "it ran great at the shop" and would not go into limp mode this morning for the mechanic. So next visit will be a new ECM more than likely.

After about four trips to the shop, they are now on the same page. They had me order the new ECM, so I went with the Delco OE unit rockauto sells. We should hear something today, and I REALLY hope this puts this issue in the past. Good grief lol

sounds like fun. I bought a used em to try to fix my no start issue, and it didn't work. hope this works for you.

Too much fun apparently, the shop called and they are bailing out, and have referred the car to a local GM dealer for the relearn procedure. Took a month to realize they were in over their heads.