Loss of CAN Bus Communication Code (U0073)
Problem:
I have twice intermittently experienced a complete loss of CAN bus communication within approx 1.5k-2k miles of driving moderately. Vehicle resumed normal operation without any intervention in less than five minutes after both occurrences.
Both occasions with stock 'Diagnostic' configuration settings, a 'Stable Traction Control' and 'ABS' announcement on the DIC was activated with audible chime, no other announcements or warnings.
On both occasions I was logging HPtuner scanner and lost communication when the incidents occurred. I have an AutoMeter fuel pressure gauge, it was bouncing around erratically both times above the minimum ECM/FPCM fuel pressure settings. Engine operation seemed normal during the occurrences prior to engine shutdown, with exception of the fuel pressure bounce. Upon engine shutdown, I experienced 'no crank' when I made attempts to restart the engine, I was also unable to reestablish HPtuner interface. One last thing, the parking brake solenoid would only actuate when I operated the brake switch, would not stay in locked position.
(See HPtuner diagnostic code screenshot images)
Troubleshooting:
After the first failure I replaced the E67/ECU. I am running a 2 bar OS.
Both times I verified the CAN bus wiring/connectors for continuity and/or physical damage, negative.
I share the following:
A CAN bus communication loss activates code (U0073). There are #2 each 120ohm resistors in a CAN bus network that read 60ohms in a healthy circuit. To perform this test a multimeter is placed across pins #4 and #16 at the OBD2 with vehicle power off. This is just one troubleshooting test, there are other tests as well.
The 120ohm resistors in a CTS-V are located at the following locations:
A resistor is located internally in the ABS/BCM, and a resistor is located internally in the FPCM/BCM.
EDIT: The 120ohm resistor may actually be located external from the FPCM/BCM according to the attached schematic. I'm still looking for it.
(see attached schematic and BCM PN images)
Also note, there is a CAN bus network connector located in proximity to the washer fluid container and potentially susceptible to washer fluid contamination/corrosion adjacent to the ABS/BCM.
The ABS/BCM is installed directly onto the "Brake Pressure Module Valve" located on the RH lower front side of vehicle forward of the wheel well, the ABS/BCM may also be susceptible to environmental and/or washer fluid contamination/corrosion due to location.
I inspected both the CAN bus connector and ABS/BCM for external signs of washer fluid and/or environmental contamination/corrosion. I then inspected connector pins common to both the network connector plug and the ABS/BCM module, I also removed the ABS/BCM from the brake master to inspect the internals, negative.
I then inspected the FPCM/BCM mounted in the trunk located on the RH sidewall for any signs of wire/pin/plug damage, negative.
Conclusion:
My main troubleshooting focus is primarily the ABS/BCM and FPCM/BCMs due location of the 120ohm resistors within the CAN bus network. Failure of either one of these BCMs could potentially cause a network failure due to the location of the 120ohm network resistors, causing a U0073 situation. I can't be 100% sure if a resistor is intermittently failing, or if some other internal BCM component is failing and/or negatively affecting the resistor performance? I'm limited to not having electrical drawings available. I'm leaning towards the ABS/BCM failing due to the fact there were no FPCM/BCM codes recorded in diagnostics, and the fact each occurrence occurred separately using different ECMs. Is it even possible the engine would continue to operate normally with an active ECM fail?
Does anyone have past FPCM/BCM and/or ABS/BCM failure information to share? I'm interested to learn if the ABS/BCM can fail intermittently over time prior to catastrophic permanent failure? Has anyone experienced similar DIC warnings and codes prior to an FMCM/BCM or ABS/BCM failure?
I hate throwing parts at a problem, this might actually be one of those diagnostic jobs fit for a dealer!
