U0009 High Speed CAN Communication Bus (-) shorted to Bus (+)

If you have a vehicle with a modern computer system, you may encounter various diagnostic trouble codes (DTCs) that indicate problems with your vehicle’s performance. One of these codes is U0009, which means that there is a communication issue with the high-speed controller area network (CAN) bus.

The CAN bus is a network of electronic control units (ECUs) that communicate and share information with each other. The CAN bus has two wires: CAN high and CAN low. When the CAN bus is in idle mode, both wires carry 2.5 volts. When data bits are being transmitted, the CAN high wire goes to 3.75 volts, and the CAN low wire drops to 1.25 volts, creating a 2.5-volt differential between the wires.

The U0009 code indicates that the CAN high wire is shorted to the CAN low wire, causing a loss of communication between the ECUs. This can affect various vehicle functions, such as engine performance, fuel economy, transmission shifting, etc.

Symptoms of U0009 Code

Some of the common symptoms of the U0009 code are:

• Check engine light illumination
• Reduced engine power
• Hard or delayed shifting
• Poor fuel efficiency
• Difficulty starting the vehicle
• Engine stalling or shutting down

Causes of U0009 Code

There are several possible causes of the U0009 code, such as:

• Faulty or damaged control module
• Open or shorted wiring or connectors in the CAN bus circuit
• Corrosion or water intrusion in the CAN bus circuit
• Faulty or loose ground connection
• Faulty or weak alternator or battery
• Interference from other electrical devices

How to Diagnose and Fix U0009 Code

1. To diagnose and fix the U0009 code, you will need a scan tool that can read and clear DTCs, a digital multimeter, and a wiring diagram of the CAN bus circuit. Here are some steps you can follow:
2. Connect the scan tool to the vehicle’s diagnostic port and read the DTCs. Note down any other codes that may be present along with U0009.
3. Clear the codes and test drive the vehicle to see if they return. If they do not return, it may be an intermittent issue that needs further monitoring.
4. If the codes return, inspect the wiring and connectors of the CAN bus circuit for any signs of damage, corrosion, or loose connections. Repair or replace any faulty components as needed.
5. Check the voltage and resistance of the CAN bus wires using the multimeter and the wiring diagram. The voltage should be around 2.5 volts when idle and vary between 1.25 and 3.75 volts when transmitting data. The resistance should be around 60 ohms when measured between the CAN high and CAN low wires at any point in the circuit.
6. If the voltage or resistance readings are out of range, there may be a short or an open in the circuit. Locate and repair the source of the problem using the wiring diagram.
7. Check the ground connection of the control modules and ensure that it is clean and tight.
8. Check the alternator and battery output and ensure that they are within specifications.
9. Check for any interference from other electrical devices, such as aftermarket radios, amplifiers, etc., that may be affecting the CAN bus communication. Disconnect or isolate any potential sources of interference and see if the code clears.
10. If none of the above steps resolve the issue, you may have a faulty control module that needs to be replaced or reprogrammed.

Conclusion

The U0009 code is a serious issue that can affect your vehicle’s performance and safety. It is important to diagnose and fix it as soon as possible to avoid further damage to your vehicle’s components. You can use this article as a guide to help you troubleshoot and repair this code, but if you are not confident or experienced enough, you should seek professional help from a qualified mechanic.

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