U0004: CAN Bus High Voltage Low
The U0004 code indicates that the ECU has detected a low voltage condition on the CAN High bus line, typically below 1.5V or 2.0V depending on the manufacturer. This suggests a short to ground on the CAN High wire, an open circuit on the CAN Low wire causing an imbalance, or a faulty CAN transceiver within a module that holds the line permanently dominant, preventing communication between modules.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Short to ground in the CAN High wiring
- Open circuit in the CAN Low or CAN High wiring
- Damaged, corroded, or loose connectors on the data bus
- Faulty CAN transceiver inside a control module (ECU, TCM, ABS, BCM, etc.)
- Missing or damaged 120-ohm termination resistors on the CAN bus
- Severe electromagnetic interference (EMI) or poorly shielded aftermarket accessories
3. Symptoms You Will Notice
- Check Engine Light or other warning lights illuminated
- Vehicle may not start or runs poorly
- Multiple warning systems inoperative (ABS, airbag, transmission, etc.)
- Communication loss with scan tool or intermittent connection
4. Step-by-Step Diagnosis
Use a scan tool to confirm U0004 and check for related codes
Inspect CAN bus wiring and connectors for damage, corrosion, or loose terminals
Measure voltage on CAN High and CAN Low with a multimeter (key on, engine off) – CAN High should be ~2.5V, CAN Low ~2.5V; check for short to ground or open
Check CAN bus termination resistors (120 ohms each) at the OBD2 connector pins 6 and 14
Isolate modules by disconnecting them one at a time while monitoring bus voltage to identify a faulty module
5. Frequently Asked Questions
Is it safe to drive?▼
No, driving with U0004 is not recommended as it can cause unpredictable behavior, loss of critical systems (ABS, airbags, transmission control), and may leave you stranded. Have the vehicle towed to a repair shop.
How much does it cost to fix?▼
Repair costs vary widely: simple wiring repairs may cost $100-$300, while replacing a faulty module can range from $500 to $1,500 or more, including diagnosis and labor.