TLDR: Does anyone know what "Class 2 Circuit Shorted to Battery" and "Class 2 Circuit Shorted to Ground" imply?


Background: Car had sat for months and drained battery, been jump started 3 times. Then thoroughly washed with a somewhat high pressure hose at a commercial self-wash place. Then put new wheels/tyres on.

Was driving around the other night after putting the new wheels/tyres on and suddenly experienced the following:

• traction control light
• ABS light
• message "shocks inoperative"
• message "service vehicle soon"
• message"reduced power"
• message "trac/act hndlg – off"
• message "fuel low"
• fuel gauge pointing to zero even though the tank is full
• Chime

Made it home, had to park the car for a few days. Troubleshooting today.

Battery was nearly dead again. Jumped from spare battery. Cleared codes. Went for drive in 80-100F heat. Symptoms:

• With door ajar to put interior lights on, press brake pedal. Interior lights momentarily dim.

Driving fine for 50min, then under heavy throttle:

• traction control light
• ABS light
• Right airbag light
• Sound system fully inoperative
• Aircon stopped cooling but kept blowing
• message "shocks inoperative"
• message "service vehicle soon"
• message"reduced power"
• message "trac/act hndlg – off"
• message "fuel low"
• fuel gauge full low
• temp gauge full hot
• Oil pressure gauge full low
• Voltage gauge ~13.8V
• Chime
• Engine lugged and chugged badly a couple of times, mostly under heavy throttle. Felt like momentary loss of fuel.

Turned the aircon off to minimise electrical load and made it home.

Codes are:
10-PCM
P1626 Theft Deterrent System Fuel Enable Circuit - H

28-TCS
U1016 Loss of Communications with PCM - H
U1301 Class 2 Circuit Shorted to Battery - H

40-BCM
U1016 Loss of Communications with PCM - H
U1096 Loss of Communications with IPC - H
U1255 Serial Data Line Malfunction - H

58-SDM
U1301 Class 2 Circuit Shorted to battery Refer to Data Link Connector System - H

60-IPC
U1016 Loss of Communications with PCM - H
U1040 Loss of Communications with TCS - H
U1056 Loss of Communications with RTD - H
U1064 Loss of Communications with BCM - H
U1088 Loss of Communications with SDM - H
U1160 Loss of Communications with LDCM - H
U1176 Loss of Communications with RFA - H
U1255 Serial Data Line Malfunction - H

80-Radio
U1016 Loss of Communications with PCM - H
U1064 Loss of Communications with BCM - H
U1096 Loss of Communications with IPC - H

99-HVAC
B0361 Left Actuator Feedback Short to GND - H
B0363 Left Actuator Feedback Open - H
B1016 Loss of Communications with PCM - H
U1064 Loss of Communications with BCM - H
U1096 Loss of Communications with IPC - H
U1160 Loss of Communications with LDCM - H
U1255 Serial Data Line Malfunction - H

A0-LDCM
U1064 Loss of Communications with BCM - H

A6-SCM
U1255 Serial Data Line Malfunction - H
U1300 Class 2 Circuit Shorted to Ground - H

B0-RFA
C2120 TPM System Malfunction (No Sensors Received) - C, H
U1016 Loss of Communication with PCM - H
U1064 Loss of Communication with BCM - H
U1096 Loss of Communication with IPC - H
U1255 Serial Data Line Malfunction - H

So pretty obviously the whole comm's network failed, at least briefly. The only current code is the TPMS, which is fine as they aren't installed. Also the aircon left actuator is bad but it's been like that for ages and presumably a separate issue.

Possibilities:
- low voltage / bad battery. Suspect most likely as fault was transient and either under headlights use or heavy throttle = higher load from fuel pump. The battery was previously drained. The battery is a AAA roadside replacement, CCA @0F of 800A, and RC of 115 minutes.
- bad comm’s port on module, or bad module
- bad connection (loose, water etc)

Strategy:
Will replace the battery and go from there.

Does anyone know what "Class 2 Circuit Shorted to Battery" and "Class 2 Circuit Shorted to Ground" imply? Those are the only codes I don't understand.

 

If the GM Class 2 network is shorted to ground or power you will see all the things you mentioned…class 2 is a single wire network that our Corvette modules communicate with one another… all of those wires come together at the 2 “star connectors” or splice packs near the BCM…you can check the class 2 network for a short to power or ground at the splice packs or at pin 2 of the DLC (where you hook up a scan tool to)…with a scope you will see a 0-7 volt square wave…a short to power will show 12 volts there…a short to ground will show ground there…close to 0 volts…what happens if a module (BCM, HVAC,PCM) loses a power or ground that power or ground will back feed onto the serial data wire and S$$T hits the fan !!…if you have a DVOM it’s easy to troubleshoot !!…video enclosed !!…a lot of times the connector inside the rubber boot on the drivers door to body pillar connection is bad and that is where some members have issues…when you see all that craziness shake the rubber boot and see if the problem goes away !!

 

Thanks for the tips, that probably is broadly the problem. In short it occurred twice and never came back (so far...), I think it might have been caused by water ingress somewhere, possibly at the base of the windscreen as the seal wasn't sitting properly and I gave it a big wash just beforehand. Will get that fixed and see if it ever comes back before pulling the car to pieces. The other possibility is low / erratic voltage from an underused battery which is now driven regularly enough to be working fine.

 

Thanks for the suggestions. Luckily, the problem has not reappeared since! I suspect it was caused by intermittent low battery voltage caused by a poorly charged battery returning to proper health after a long period of neglect. Another possibility could be water intrusion at the base of the windscreen due to the seal not being seated properly.

Will monitor and see how it goes. Your suggested troubleshooting guide seemed accurate and on the right track as a place to start.

 

The water intrusion is because of clogged drain “utters” at the base of the windshield cowl…I’d see if they are clogged (a few YouTube videos on this)…water gets into the car near the BCM and you may notice that area damp after a heavy rain !!