Tech Tips (by ALLDATA): Buick LaCrosse instrument cluster unstable & engine stalls

Problem: The vehicle owner stated that while driving or while sitting at a stop, the warning lights in the instrument cluster start flashing, the tachometer and speedometer gauges fluctuate erratically, and the engine starts to surge then will eventually stall out.

Case Details: The technician verified the customer’s complaint with the vehicle sitting and idling in the service bay. When he scanned for diagnostic trouble codes (DTCs), he found the following:

• B101E 43 – Electronic Control Unit Software EEPROM Incorrect Programming
• U0020 – Low Speed CAN Communication Bus Performance
• U0073 – Control Module Communication Bus Off
• U0074 – Control Module Communication Bus B Off
• U2099 5A – High Speed Communication Enable Circuit Not Plausible

Not having a lot of experience with network code diagnostics, the technician called ALLDATA Tech-Assist for some guidance.

The Tech-Assist consultant advised him that, the only module that was included in all the bus networks (high-speed, low-speed, and communication enable circuits), and could have that kind of impact to both the instrument cluster and PCM, was the body control module (BCM).
The consultant further noted that the BCM could be at fault, but judging by the DTCs, another module most likely was failing and causing the BCM to not communicate properly.

He offered the technician two different diagnostic strategies.

1. The technician could determine which module set the B101E 43 code and follow the factory GM diagnostics for it (available in ALLDATA).
2. The other strategy would be to wait for the system to act up and not communicate, then start unplugging modules from the bus one at a time (starting with the low-speed bus) until communication was restored. Junction connector JX200 was identified as the most convenient place to access all the modules on the low-speed bus.

The technician decided to use the second diagnostic option and found that with the inflatable restraints sensing and diagnostic module (SDM) unplugged, communication was restored, and all the issues were resolved.
Confirmed Repair: The technician verified that all the power, grounds, and connections to the SDM were okay. He replaced the SDM (required programming), cleared the DTCs and road tested to verify the repair. Fixed!