Help! AT Check light, and after restarting I get a U1000 code + CEL
 

Hey - I just bought a 2005 G35 Sedan w/ 145k miles in pristine condition a few days ago, but I've had some strange problems since. Twice now the AT Check light has come on & put the car into a pseudo-limp mode (stuck in a specific gear), and it only disappears after a restart. But after the restart, the service engine soon light comes on instead & checking with a code reader gives me a U1000 code with no description. Car has been fantastic aside from this, but it's extremely worrying because it seems like this is a transmission or engine issue. Reading up on it gave me hope that it could be electrical but I really need some help - I'm pretty short on money so I'd like to try and diagnose/fix the issue myself.

Aside from that, the engine also sounds odd compared to my previous car but I have no clue if it's normal. Two short videos attached.

Extremely worried the car has major problems, but I don't get why that would be because there's extensive maintenance record and it doesn't have more than a few scratches on it.

Thanks in advance!

My last car was a shitbox with 310k miles so I'm just used to every problem being a major one with an expensive fix, hopefully thats not the case here :/

OBD2 scan for what transmission code is popping up, however U1000 is a generic (some computer somewhere got unplugged) so I would suspect that the TCM is losing power for whatever reason.

Look on the driver side of your transmission, are there 2green 1black harness coming out of the transmission mounted to the side of the casing, or is there 1 large grey harness plugged into the transmission?

If you have the 3 harnesses your TCM is located in the passenger footwell next to the ECM, if you have the 1 harness it's located inside the transmission behind the valve body.

Regardless of which style you have I would disconnect the battery, unplug those harnesses and inspect for any damage, corrosion to the pins, etc.

Also check your transmission GROUND WIRE which is located on the top middle of the engine timing chain cover, there are two wires landed on GREEN BOLTS. Unbolt, use a wire brush to clean up the lug, bolt, threads, use a little electrical anti-oxidation grease (not dielectric), and bolt it back together.

My ODB2 scanner didn't pull up any transmission codes unfortunately because I had to turn off the car and restart it to clear the transmission problem (I was like 50 miles from my house). I'll definitely check out the wiring though, especially since some of the ground connections in the engine bay were severely corroded. I ordered a higher amperage/thicker wire grounding kit as well since some of the wiring in the crimped-on connectors is incredibly oxidized & all you can see is green.

Thanks for the suggestions, really appreciate the help! I'll check everything out in a little bit.

Your handheld scanner might not pull transmission codes, check to make sure.

You should definitely check the engine to chassis bonding jumper as well, it's on the right side (passenger) of the timing chain cover, larger AWG wire, it lands on the chassis directly below the coolant reservoir on a 2-hole lug. You don't need to disconnect the coolant reservoir, just unbolt the single 10mm bolt, unhook the hose from it's keepers, and lift the reservoir straight up, there's enough slack on the hose to relocate it to access the lug below it.

Wire brush, de-ox, bolt it back together, both ends of the cable. Also hit both ends of the battery negative cable. You will need to remove the plastic cover (not just the access panel that pops out) to get to the other end of the battery negative which is on a 2-hole lug bolted to the chassis. The cable is only about 12" long total.

Might as well hit the positive terminal, fusible link connections as well while the battery is disconnected.

I prefer ILSCO de-ox, but Ideal brand NOALOX or Gardner Benders OX GARD all work well. Those are the three industry standard anti oxidation grease for electrical work, your local Home Depot will carry one of the three.

https://cimg4.ibsrv.net/gimg/g35driv...001dbe2644.jpg
Would that happen to be this wire this is right under the coolant reservoir? I'm not quite sure if this is what you mean, but looks like this is what you're talking about. There's a power steering fluid leak (only a slight one) so it's covered in gunk and that could definitely be a bad connection. The other ones that connect up to the front engine timing cover were pretty oxidized so I wire brushed those. Thanks for the tips! Gonna try that today as well.

Yep that's it, when you lift up the coolant reservoir you'll see it's a 2-hole lug. Wire brush until you have shiny metal again on all mating surfaces. The BOLT ITSELF is what makes the connection on those chassis connections, the surface directly below the lug is painted so the current flows through the top of the lug, into the bolt head, through the bolt threads, and into the chassis threads.

Personally I use a burr bit on a drill and remove a little paint so the lug clamps down on bare metal, just slather on the DEOX so it won't corrode/rust.

This is very late but thanks a lot for the help! I got a grounding kit & wire brushed everything (de-ox coming soon as well) and the transmission issue hasn't come back since! 🥳🥳🥳