Quick Answer
U0073 means your vehicle’s primary communication bus (CAN Bus A) between control modules has lost connection. The most common fix involves checking battery voltage, inspecting wiring harnesses for damage, and testing the CAN bus termination resistors. This is a serious code that often prevents the vehicle from starting or operating normally.
What Is OBD Code U0073?
Code U0073 indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a loss of communication on Control Module Communication Bus A—commonly known as the CAN (Controller Area Network) bus. This is the primary electrical communication highway that allows all of your vehicle’s control modules (engine, transmission, ABS, body control module, etc.) to share critical data and coordinate operations.
When this bus goes offline, the modules cannot communicate with each other, which typically results in a no-start condition, limp mode, or multiple warning lights on the dashboard. This is not a minor issue and should be diagnosed promptly.
What Does U0073 Mean?
The U-series codes (U0000–U0299) are network communication codes defined by the Society of Automotive Engineers (SAE). Specifically:
- U0073 = Control Module Communication Bus A Off
- Bus A = The primary CAN bus (typically 500 kbps in most vehicles)
- “Off” = The ECM/PCM is not detecting any signal activity on the bus
The CAN bus operates at a specific voltage level (typically 2.5V nominal when idle, switching between 0V and 5V during communication). If the ECM detects no bus activity or voltage outside normal parameters for a set duration, it logs U0073.
Common Symptoms
- Engine will not start or cranks but won’t fire
- Multiple warning lights illuminated (Check Engine, ABS, Airbag, Traction Control, etc.)
- Vehicle stuck in limp mode with reduced power
- Transmission stuck in one gear or won’t shift
- Instrument cluster displays are blank or flickering
- Power windows, locks, or other convenience features not responding
- Dashboard gauges behaving erratically
- Complete loss of vehicle communication (no response to key fob)
Possible Causes (Most to Least Common)
1. Dead or Weak Battery (Most Common)
A battery voltage below 9.6V can prevent proper CAN bus communication. Even if the engine cranks, insufficient voltage starves the modules of power needed to maintain the bus signal.
2. Corroded or Loose Battery Terminals
Poor battery connections create high resistance, effectively reducing available voltage to the modules. This is one of the easiest fixes.
3. Damaged CAN Bus Wiring Harness
The CAN bus uses two twisted wires (CAN High and CAN Low). Damage from accidents, rodents, or corrosion can break the circuit or cause shorts.
4. Faulty CAN Bus Termination Resistors
Each end of the CAN bus has a 120-ohm termination resistor. If these fail or become disconnected, the bus cannot function properly.
5. Defective Gateway Module or Splice Pack
Many vehicles use a gateway module or splice pack that connects different CAN bus segments. Failure here can isolate the entire network.
6. Faulty Engine Control Module (ECM) or PCM
While less common, a defective ECM/PCM with a failed CAN transceiver chip will prevent bus communication.
7. Blown Fuses Related to CAN Bus Power
Dedicated fuses supply power to the CAN bus transceivers. A blown fuse will disable communication.
8. Aftermarket Alarm or Remote Start Installation Issues
Improper installation can damage CAN bus wiring or create electrical conflicts.
Diagnostic Steps
Step 1: Check Battery Voltage
Use a multimeter to measure battery voltage with the key off. It should read 12.6V or higher. If below 12V, charge the battery and retest. With the engine running, voltage should be 13.5–14.5V.
Step 2: Inspect Battery Terminals and Connections
Look for corrosion (white, blue, or green crusty buildup) on battery posts and cable terminals. Clean with a wire brush and baking soda solution if corroded. Ensure connections are tight.
Step 3: Check CAN Bus Fuses
Consult your vehicle’s fuse diagram (usually in the owner’s manual or under the hood cover). Look for fuses labeled “CAN,” “Gateway,” or “Module.” Replace any blown fuses with the correct amperage.
Step 4: Inspect CAN Bus Wiring
Locate the CAN bus connectors (often on the gateway module, firewall, or under the dash). Look for:
- Visible damage, cuts, or abrasions
- Water intrusion or corrosion
- Loose or corroded connectors
- Pinched or crushed wires
Step 5: Test CAN Bus Voltage with a Scope or Multimeter
A digital oscilloscope is ideal, but a multimeter can work. Measure voltage between CAN High and CAN Low lines. During idle, you should see approximately 2.5V. During communication, you’ll see oscillating signals. Zero or 5V steady indicates a problem.
Step 6: Check Termination Resistors
Using an ohmmeter, measure resistance between CAN High and CAN Low. You should read approximately 60 ohms (two 120-ohm resistors in parallel). If open or shorted, the resistors need replacement.
Step 7: Scan for Additional Codes
Use a professional diagnostic scanner to check for related codes like U0100, U0101, U0102 (which often accompany U0073). These can point to specific modules that lost communication.
Step 8: Test the ECM/PCM
If all wiring and power checks pass, the ECM/PCM may have a faulty CAN transceiver. This requires professional testing or replacement.
Repair Cost Estimates
| Repair Type | Estimated Cost |
|---|---|
| Battery replacement | $100–$300 |
| Battery terminal cleaning/replacement | $50–$150 |
| CAN bus fuse replacement | $20–$100 |
| CAN bus wiring harness repair/replacement | $200–$800 |
| Gateway module replacement | $300–$1,200 |
| ECM/PCM replacement | $800–$2,500 |
| Professional diagnosis | $100–$200 |
Note: Costs vary significantly by vehicle make/model, location, and whether you use a dealership or independent shop. Labor typically ranges from $75–$150 per hour.
Can I Still Drive?
Severity: HIGH — Do not drive the vehicle.
U0073 is a critical communication code that typically prevents normal vehicle operation:
- No-Start Condition: In many cases, the vehicle won’t start at all because the ECM cannot communicate with the fuel injectors, ignition system, or transmission.
- Limp Mode: If the vehicle does start, it may be stuck in limp mode with severely reduced power, making it unsafe for highway driving.
- Loss of Safety Systems: ABS, airbag, and stability control systems may be offline, eliminating critical safety features.
- Unpredictable Behavior: Without module communication, the vehicle may shut down unexpectedly or behave erratically.
Recommendation: Have the vehicle towed to a repair facility. Attempting to drive it risks stranding yourself, causing an accident, or causing further damage.
FAQ
Q: What’s the difference between U0073 and U0074?
A: U0073 refers to Communication Bus A (the primary CAN bus), while U0074 refers to Communication Bus B (a secondary CAN bus used in some vehicles). Both indicate loss of communication on their respective buses. The diagnostic steps are similar, but the wiring locations differ.
Q: Can a bad alternator cause U0073?
A: Yes, indirectly. If the alternator fails, battery voltage drops during driving, which can cause the CAN bus to lose signal. However, a bad alternator typically triggers charging system codes first (like P0562 or P0563). Always check battery voltage and charging output when diagnosing U0073.
Q: Will clearing the code fix the problem?
A: No. Clearing U0073 with a diagnostic scanner will only temporarily erase the code from memory. The underlying problem (battery, wiring, module) must be repaired. The code will return immediately or after a few drive cycles if the root cause isn’t addressed.
Q: Can I replace the ECM myself to fix U0073?
A: Not recommended without professional equipment. While ECM replacement is possible for experienced DIYers, most modern ECMs require programming/flashing with the vehicle’s VIN and security codes. Dealerships or professional shops have the necessary diagnostic equipment and software. Improper installation can worsen the problem.
Q: Is U0073 covered under warranty?
A: It depends on your vehicle’s age and mileage. If the vehicle is still under manufacturer warranty (typically 3 years/36,000 miles), CAN bus communication issues are usually covered. Battery and wiring repairs may be covered as well. Check your warranty documentation or contact your dealership.