What it means
Bus B low-side line voltage below expected range. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
At a glance
- Usually lights the warning light of the system that lost communication
- The engine or gearbox may go into limp mode
How it works
Bus B uses paired conductors to exchange messages between its connected modules, and the (−) conductor is the low-side line. U0043 is set when this negative line remains below its expected voltage range, which may result from a short to ground or another fault pulling it low.
Common causes
Most likely first.
- Wire shorted to ground
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Record all codes and freeze-frame data, then identify which bus B modules report communication problems and when they occur.
- Inspect bus B (−) wiring and connectors for chafing, crushed insulation, exposed wire touching ground, corrosion, or loose pins.
- Use scan-tool live network status to determine whether the fault affects the whole bus or appears when a particular module or harness section is involved.
- Following the manufacturer's test procedure, measure bus B (−) behavior and check the conductor for a short to ground.
- If the wiring checks pass, isolate suspect modules and test gateway routing and bus termination against the manufacturer's specifications.
Common fixes
- Repair exposed or chafed bus B (−) wiring shorted to ground.
- Secure or replace damaged connector terminals on the negative line.
- Repair a bus B splice or harness section with poor continuity.
- Repair a module or gateway only after testing confirms it is pulling the line low or routing messages incorrectly.
Can I keep driving?
If communication loss affects an essential driving system, stop and arrange recovery. If only secondary functions are affected, drive cautiously and get the fault checked soon because more bus B modules may become unavailable.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.