2009 2.2L LT with ~82,000 miles, have had it for about a year but started having problems all of the sudden with all the gauges dropping down to zero and the warning lights turning on and off while driving. It drives fine when it happens, no hard shifts or anything like that.

-Car would sometimes not respond to key fob and would not start. The car would turn on, but no crank. I was able to get it to start when shifting from Park to Neutral, but it is not a consistent solution
-Based off Vin, it shows that the recalls were fixed.
-Was able to clean some of the ground locations (G103, G107, G109, G203, G201) as well as the battery terminals but all were tight and had little corrosion to begin with.
-Did the loosen and tighten the bolts trick in the fuse box, even disconnected it and cleaned the connections with connector cleaner but no major signs of damage/burns. Also made sure that the bolt on the big red connector near the glove box was tight and the pins all the way in (something I saw from the Cobalt forums).
-I tested the battery with an Ancel battery tester, and it shows that the battery and alternator are "good" (Loaded 12.35V, Unloaded 14.54V for charging test, but at rest was 12.24V. I even charged the battery but it wouldn't go past 12.5V). My good ol RadioShack voltmeter showed 12.5 at rest and 14.64 when running. So seeing that both my scan tool (Launch X431) and the battery tester were showing 12.2-12.4 range, I tried to get a new battery. But at the auto parts store, they did their test and recorded: 14.77 no load, 14.68 loaded, with a ripple voltage of 24mV, and 12.7V at rest.
-I tested all of the smaller fuses for continuity and all passed, could only do a visible check on the larger fuses because they are hard to get out and I don't want to break them. I also checked the relays with a 9V battery and they work.
-I tested G103, G107, G108 and G109 and are all 12.52-54V

BCM codes: U2100, U2125, U2172, B1517-5A
IPC codes: B1325-03, U2107, U2129
Radio codes: U0140, U0155
EBCM: C0035 (I replaced the wheel bearing last year and this code comes and goes)
I get no codes from the ECM or TCM. After clearing codes, U2125 and U2172 are left.

My car does have OnStar, and I have read that the connection/wires at the OnStar module could be the culprit but I do not want to go through the hassle of that just yet. I did pull the fuse for the OnStar/XM but no change. Anything else I should check before I give up and bring it to a shop? I do have a relay tester coming in to make sure that the relays are actually good. But it definitely seems like a bad ground/wire somewhere...

 

You can test relays without buying a tester. The OnStar is known for can bus issues. You will want to delete it and see.

 

Welcome to the forum, let’s start real simple, tighten the 4 7 mm bolts in the under hood fuse box, the firmly push into place all the fuses and relays , then the same to all the relays and fuses in the interior fuse panel on the passenger side of the waterfall.

 

So I was able to get to the OnStar module in the back, was not that bad as I thought it would be, you can finagle the plastic without removing the entire panel to get at it after removing some bolts. The post I saw about removing the connector says to only remove one of the white connectors, so I did that but still had the same problems. I just ended up disconnecting both white connectors and car starts up no problem and no dashboard issues. Did a little test drive and no issues either. Only codes that continue to show up after some clears are ones related to OnStar/XM (U2125, U2172, U0194, B1325-03 and B1517-5A). Not going to call it fixed yet, but it's something at least. And yes the fuse box bolts were tightened "gorilla tight" and the fuses/relays were all pushed in way before I decided to try the OnStar module.

 

The intermittent C0035 would be a loose wire to the driver side front wheel.
The OnStar/XM module does not make that sort of problem, pulling the fuse should be good enough.
What is the radio doing? Is it after-market?
All of the U codes are related to 1 of the modules either getting no power or the canbus wire is disconnected. The canbus is like an old fashioned coax computer network with a terminating resistor on the end of the line and the computer on the other end; if a module shuts down the modules downstream shut down.

 

You don’t mention if you did as I suggested in post #3.

 

post #4 says he tightened bolts and pushed on the relays (last sentence)

 

Yea, OnStar was not the culprit, just tried to run an errand and the dashboard freaked out. Still drives fine though. The radio is stock, and the most recent work that was done was exhaust system and new O2 sensors. I did the CAN bus test that donbrew suggested, and my results are: With car off Grounds 4 and 5 connected to 16 show the battery voltage of 12.2, pin 6 and 14 show the expected 60 Ohms. With car on, pins 4 to 6 shows 2.4V, 4 to 16 2.12V and same for 5-6 and 5-16. I have not probed the BCM pins yet, but some weird stuff happened when trying to turn the car off.
1. The dashboard and lights stayed on when the care was set to off and the key removed
2. The interior lights would not turn off after the key was removed, I made sure to close all the doors but no change. I had to disconnect the battery to get the lights to turn off.
I read the codes before I started doing the CAN bus stuff and the codes were: B1517-5A, U2172, U2100, U2125, U2107, U0194, U0140, U0155. So it looks like the BCM is losing CAN bus connection, and the IPC and radio are losing connection to the BCM.

 

12.3V is low.

No point chasing electrical problems with an iffy battery. I would put in a new one. Even if the problem isn't the battery, and it often is, you at least have a battery that will last you awhile.

 

K, I see that now. And 12.3 is a low battery, it might also have a dead cell or 2