[DegaZL1]

My 2018 Camaro ZL1 (A10) has been throwing the following codes recently: P0700, U0073 and U0101.

P0700 – Transmission Control System (MIL Request)
U0073 – Control Module Communication Bus Off
U0101 – Lost Communication With TCM


All of these relate to transmission/communication issues.
The car would drive OK one day without any issues, but the next day it will throw those CEL codes again and enter limp mode without shifting gears – the transmission would hold in 3rd or 4th gear. (Limp Mode)



After driving a while under those conditions the “P” Service wrench” light also lights up (along with the CEL).


Furthermore, warnings such as “Service Rear Axle” , “Service Power Steering” and “Rear Axle System Off” would be displayed and the car would then slow down almost to a halt without responding to throttle input.


The car’s speed gauge would then stop showing the driving speed , while the tachometer goes up and down like crazy.



When I proceed to park the car and remove the battery and leave it for a while it usually prompts me to “Shift To Park”, while the car is already in P, thus not recognizing that the car is duly in Park already.


When reattaching the battery after an hour or so, usually the car would start and drive OK again overall, but then those codes will repeat again after a day or two. Since the issue is not consistent, I don't think it is hardware related.

Can you please help me diagnose this issue ?


Since it is not only TCM /communication issues but also other errors like Rear Axle Service, I am suspecting that there is a ground issue or exposed/corroded wires somewhere.


I removed the AEM Wide band from the OBD2 port as I read on the forums that sometimes the wide band can cause these issues, but the issues remain even when the wide band cable is detached from the OBD2 port.


2018 ZL1 with A10 Transmission. - ~15K Miles (Low Miles - Only Drive it in weekends)

Mods: Headers, Cat Delete, RotoFab CAI, Lingenfelter LT4 Upgrade Kit 720HP, AEM Wideband


Can you please help me diagnose this issue ?

[BMLMZ]

Silly question, but hows your battery voltage? Low voltage/weak batteries play insane tricks with these camaros and corvettes. They are super sensitive to low voltage from aging or bad batteries. I'd get a meter and see what shes at when its off. I know it seems far fetched. Its an easy thing to rule out. I went and looked at a ZL1 a few months ago. It had all those service this and that messages. Ran like poop. All kinds of issues. Dealer replaced the battery and cleared all the codes. Car ran perfect after that. A buddy of mine bought it and put 10k miles on it from that point and never had a hiccup with it. I thought it was completely screwed and passed on it...lol.

[Vigilante375]

As mentioned, I also agree that it is the battery. Modern cars are very sensitive to a battery going bad.

My 20 RAM 1500 with 55k miles recently had a lot of weird issues that was a bad battery. Rough idle, wouldn't go no more than 25 mph, engine check light on, TC light on, has to be jumped and after taking the truck to autozone, they said the battery was good! Changed the battery anyways and everything cleared up.

[Gunkk]

As others have said, these kinds of things can and will happen when modules go low voltage. I also believe it happens because the modules don't wake up nicely and can throw codes when some modules wake up before other modules, or they wake up with low voltage. Going from complete zero power to full crank-start-and-run seems to do Bad_Things(TM). My technical guess is that there is a race-condition with the CAN bus from zero power that is not normally present when the battery is connected and the modules are getting fed 12v on their "sleep" circuits.

My garage queen show car is stored power off, battery disconnected. I have seen that exact series of codes on my car a few times. Every time it has been after I've reconnected the battery after it has sat with the battery disconnect disconnected, and then started it. Shift to park and P0700 are common. Service rear axle and service stabilitrak have shown up after the battery died (before I got my tender).

Powering up in the car in maintenance/service mode avoids this. And it can also be used to "cure" the shift-to-park fault when the car gets stuck in P-code purgatory.

Here's how I do it:
0. Start with battery disconnected <-- do this if you are stuck in "shift to park" hell.
1. Connect the battery to the car (close the above linked switch).
2. Wait a minute for the car to wake up.
3. Put car into maintenance mode. Foot OFF the brake pedal, hold start button for 5 seconds. Engine will not start, but car will be KEY-ON. CEL will be on IIRC.
4. Connect OBD2 reader, poll for codes (none typically present on my car). This step is not required, but I normally do it anyway just in case it throws codes anyway. If I get any codes, I clear them here. Don't start the car with active codes, it won't like that.
5. If no codes or after codes were cleared, then start the car. Let it finish it's startup sequence (i.e. come out of fast idle down to normal idle). CEL will go off when the engine starts. If it isn't, I will re-pull codes, reclear, stop the engine and restart.
6. Go drive it or stop the engine or whatever you want. Car will be fine now.

[DegaZL1]

Originally Posted by BMLMZ

Silly question, but hows your battery voltage? Low voltage/weak batteries play insane tricks with these camaros and corvettes. They are super sensitive to low voltage from aging or bad batteries. I'd get a meter and see what shes at when its off. I know it seems far fetched. Its an easy thing to rule out. I went and looked at a ZL1 a few months ago. It had all those service this and that messages. Ran like poop. All kinds of issues. Dealer replaced the battery and cleared all the codes. Car ran perfect after that. A buddy of mine bought it and put 10k miles on it from that point and never had a hiccup with it. I thought it was completely screwed and passed on it...lol.

Thanks for your suggestion. Highly appreciated.
Battery has been tested last month and appears OK and car cranks fine too.
I will have the battery tested again elsewhere just to make sure.
Thank you.

[DegaZL1]

Originally Posted by Gunkk

As others have said, these kinds of things can and will happen when modules go low voltage. I also believe it happens because the modules don't wake up nicely and can throw codes when some modules wake up before other modules, or they wake up with low voltage. Going from complete zero power to full crank-start-and-run seems to do Bad_Things(TM). My technical guess is that there is a race-condition with the CAN bus from zero power that is not normally present when the battery is connected and the modules are getting fed 12v on their "sleep" circuits.

My garage queen show car is stored power off, battery disconnected. I have seen that exact series of codes on my car a few times. Every time it has been after I've reconnected the battery after it has sat with the battery disconnect disconnected, and then started it. Shift to park and P0700 are common. Service rear axle and service stabilitrak have shown up after the battery died (before I got my tender).

Powering up in the car in maintenance/service mode avoids this. And it can also be used to "cure" the shift-to-park fault when the car gets stuck in P-code purgatory.

Here's how I do it:
0. Start with battery disconnected <-- do this if you are stuck in "shift to park" hell.
1. Connect the battery to the car (close the above linked switch).
2. Wait a minute for the car to wake up.
3. Put car into maintenance mode. Foot OFF the brake pedal, hold start button for 5 seconds. Engine will not start, but car will be KEY-ON. CEL will be on IIRC.
4. Connect OBD2 reader, poll for codes (none typically present on my car). This step is not required, but I normally do it anyway just in case it throws codes anyway. If I get any codes, I clear them here. Don't start the car with active codes, it won't like that.
5. If no codes or after codes were cleared, then start the car. Let it finish it's startup sequence (i.e. come out of fast idle down to normal idle). CEL will go off when the engine starts. If it isn't, I will re-pull codes, reclear, stop the engine and restart.
6. Go drive it or stop the engine or whatever you want. Car will be fine now.

Thanks for your recommendation with the clear step-by-step guide.

I will have the battery tested (again) this week and then try the steps you suggested and report back.

Last month I performed a battery test and it was fine. I will have it retested elsewhere and then try the steps you suggested. Thank you.

[BMLMZ]

Originally Posted by DegaZL1

Thanks for your recommendation with the clear step-by-step guide.

I will have the battery tested (again) this week and then try the steps you suggested and report back.

Last month I performed a battery test and it was fine. I will have it retested elsewhere and then try the steps you suggested. Thank you.

Battery test is like meh. I'd just spend the $200 or so and rule it out. You can get one from walmart and return it if its not the problem. But you might be surprised.

[CalgaryZL1]

Originally Posted by DegaZL1

Thanks for your recommendation with the clear step-by-step guide.

I will have the battery tested (again) this week and then try the steps you suggested and report back.

Last month I performed a battery test and it was fine. I will have it retested elsewhere and then try the steps you suggested. Thank you.

Also, you need to get a good load test on it to make sure it is actually still good not just showing 12 V