What it means
The communication bus A low-side line (CAN-L) is open — broken wire, unplugged connector, or terminator missing.
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 the complementary side of the network signal. U0033 indicates that this conductor is open, so the modules may lose communication; a broken wire, disconnected connection, or termination fault can produce that condition.
Common causes
Most likely first.
- CAN-L wire broken or unplugged
- 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 save event data, noting which bus A modules are unreachable and whether other bus or termination codes are present.
- Inspect the bus A negative wire and connectors for breaks, unplugged connections, corrosion, or harness damage.
- Check power and grounds at modules reporting no communication to rule out a local supply issue.
- With power state set as specified by the manufacturer, check continuity of the negative conductor and test termination according to the bus layout and service information.
- If wiring and termination pass, isolate connected modules and the gateway using the manufacturer's procedure to find a node interrupting bus communication.
Common fixes
- Reconnect an unplugged bus A negative-line connector.
- Repair a broken or damaged negative conductor.
- Repair a corroded connection or restore a missing termination only if testing confirms it is faulty.
- Repair gateway routing or replace a faulty module only after isolation testing identifies it.
Can I keep driving?
An open bus A line can disable multiple systems or place affected modules in limp mode, so avoid driving if vehicle behavior or essential controls are affected. Arrange prompt diagnosis, and recover the vehicle if it cannot be operated safely.
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.