What it means
CAN-Low on the low-speed bus is shorted to battery. Bus signaling is no longer possible; comfort modules drop offline.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
The low-speed CAN bus carries messages between comfort and body-control modules. The monitoring module sets U0027 when the bus's CAN-Low conductor is held near battery voltage, preventing the two bus lines from signaling normally and potentially taking connected modules offline.
Common causes
Most likely first.
- CAN-Low shorted to battery
- 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
- Use a scan tool to record which modules are offline and the freeze-frame or event data, since a cluster of missing comfort modules can point to a shared bus fault.
- Check the battery and charging system and inspect low-speed bus connectors and harnesses for corrosion, pin damage, or contact with a power wire.
- With the vehicle powered down as specified by the manufacturer, check CAN-Low for a short to battery and compare its readings with the manufacturer's specification.
- Check bus termination only if the wiring layout and service information identify the expected terminating points; compare the measured resistance with the manufacturer's specification.
- If wiring and termination check out, isolate connected modules or the gateway one at a time using the manufacturer's procedure to find a node that is pulling CAN-Low high.
Common fixes
- Repair chafed wiring or remove a CAN-Low short to the power supply.
- Clean, secure, or repair corroded or loose bus connectors.
- Replace a faulty terminating resistor if testing confirms it is out of specification.
- Repair or replace a gateway or connected module only after circuit tests identify it as the cause.
Can I keep driving?
If multiple comfort functions are unavailable, avoid relying on affected features and arrange diagnosis soon. If the fault also disables safety-related systems or causes unpredictable vehicle behavior, stop driving and have the vehicle recovered.
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.