What it means
Bus A low-side line voltage below the expected range. Short to ground or transceiver pulling line low.
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 A's negative conductor, commonly CAN-Low on a CAN network, carries one side of the network signal. U0034 sets when its voltage falls below the expected range, which can result from a short to ground or a module pulling the line low and can disrupt communication for connected modules.
Common causes
Most likely first.
- CAN-L 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
- Scan all modules and record event data, noting which bus A nodes drop offline and whether the low-voltage condition is current.
- Inspect the bus A negative wire and connectors for chafing against ground, crushed sections, corrosion, or pin damage.
- Check battery voltage and module power and grounds, since a supply fault can affect network readings and module communication.
- Measure the negative line under the manufacturer's specified conditions and check for a short to ground against service information.
- If the harness passes, isolate connected modules and gateway routing using the manufacturer's procedure to locate a node pulling the line low.
Common fixes
- Repair a bus A negative wire shorted to ground.
- Repair chafed wiring or a damaged harness section.
- Clean or replace a poor connector or terminal.
- Repair gateway routing or replace a faulty module only if isolation testing confirms it is pulling the line low.
Can I keep driving?
Multiple bus A systems may become unavailable or enter limp mode, so avoid driving if vehicle control is affected. Stop and arrange recovery if essential or safety-related functions are unreliable.
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.