David Dauenheimer

I have a 2017 Tahoe 70k miles. Suddenly experienced weird shifting issues (hard shifts, loss of power) check engine light went on. Seemed to work better if I turned traction control and stabilitrak all the way off and manually shifted. Took it to Autozone and got codes p700 and p741. Took it to a mechanic and they said I need a new transmission. Took it to a good transmission shop and they said the transmission was fine but that I had a computer issue that was causing it to shift weirdly. Took it to dealer and they said new transmission needed. Took it to another mechanic and they cleared the codes, got it to repeat the problem, and then found the following codes: P700 TCM control module requested illumination; P741 Torque Converter Clutch system stuck off; U73 Control module communication bus A off; u77 Control module communication chassis expansion CAN bus off; u100 Lost communication with engine control module; u101 Lost communication with transmission control module; u126 Lost communication with steering wheel angle sensor module; u73 Control module communication high speed CAN bus off; u422 Invalid data received from body control module; B1325:03 Control module power circuit low voltage; u150F LIN bus; u 415 Invalid data received from electronic brake control module. They then recommended a new transmission.

I'm thinking this still sounds like a computer issue? I'd hate to spend $6k on a transmission I don't really need? I've read that sometimes low battery or alternator problems could lead to computer issues and am thinking of dropping a new battery in as mine is pretty old and should probably be replaced anyway. Guessing that will not likely magically fix everything though am looking for thoughts from folks with more wisdom.

Thanks!

Gumby22

There are lots of possibilities for the cause of these codes, but I need to know what engine/transmission it has and whether or not it’s 4x4.

If you’re not sure, check the RPO label in the glove box - the 3 digit codes that start with L and M are the ones that identify the powertrain.

The short and sweet answer to the battery/alternator question is yes, they can cause problems with modules if they’re faulty - but unless you’re experiencing other electrical problems (such as hard starting) then it’s unlikely to be the cause.

I can provide more specifics when you post the powertrain info.

David Dauenheimer

It's 4wd and the codes look like L83 MAH MYC.

Gumby22

Hold please.

My internal CPU hasn’t been updated since the days of 3.5” hard disks and Windows 95….

Gumby22

L83 = 5.3 w/ VVT & AFM

MYC = 6L80

MAH is a marketing area/destination code. Go Here for a complete list of all the RPO definitions - it’s not my website but I found it some time ago.

P0741 indicates the TCC either isn’t locking up completely or it’s not locking up at all. The most likely ​​​​​​ cause is the solenoid (sticking/binding, wiring issues or internal failure).

That said, the communication codes should first be pursued, as [some] comm errors can affect module operation.

Check the integrity of the CAN circuit by checking resistance between pins 6 & 14 at the OBD connector under the dash. If it reads anything other than 60 ohms, the network has a problem.

David Dauenheimer

It reads 61.1.

Gumby22

That’s close enough to be sure the network is in tact - I should’ve mentioned a tolerance of 1-2 ohms was acceptable (or whatever your meter leads may have for resistance).

Something I often forget about is the primary electrical connection for the transmission is prone to getting ATF inside it due to leaking seals. This can cause shorting (hard faults or intermittents) between the circuits and affect transmission operation.

At the right rear of the transmission is the connector. You may need to remove the heat shield for accessibility, but once disconnected, inspect for ATF inside it. If found, hit both sides with electronic contact cleaner and compressed air, then reconnect and test drive.

If the concern is no longer present after cleaning the connectors, that’s the source of the problem.

If it’s still present, make a note that the internal transmission harness will need replacement once the cause is diagnosed.