Introduction:
The U0013 Diagnostic Trouble Code (DTC) indicates a fault within the Medium Speed CAN (Controller Area Network) communication bus, specifically that the CAN (+) line is registering a voltage lower than expected. This bus is crucial for various electronic control units (ECUs) within a vehicle to communicate and share data. A U0013 code signals a potential disruption in this communication, which can lead to a variety of symptoms and performance issues.
Table: U0013 Medium Speed CAN Communication Bus (+) Low
| Category | Description | Potential Causes |
|---|---|---|
| Definition | U0013: Medium Speed CAN Communication Bus (+) Low. Indicates the CAN High wire in the Medium Speed CAN bus has a voltage lower than the expected range. | Short to ground in the CAN High wire, faulty ECU pulling down the CAN High voltage, wiring harness damage, corroded connectors, incorrect termination resistance, faulty CAN transceiver. |
| Symptoms | Loss of communication between ECUs, malfunction of various vehicle systems (ABS, TCS, Transmission, etc.), warning lights illuminated (ABS, Traction Control, Check Engine), limp mode, no start condition, erratic gauge readings. | Vehicle may exhibit a wide range of problems depending on which ECUs are affected. Symptoms can be intermittent or constant. |
| Troubleshooting | Verify the DTC, check for other CAN bus related DTCs, inspect wiring and connectors for damage/corrosion, measure CAN High and CAN Low voltages, check termination resistance, isolate ECUs to identify the faulty unit, use a scan tool to monitor CAN bus data, perform a wiggle test on wiring harnesses. | |
| CAN Bus Basics | Differential signaling using two wires (CAN High and CAN Low), utilizes termination resistors at each end of the bus (typically 120 ohms each, resulting in 60 ohms total resistance across the bus), ECUs communicate by transmitting messages on the bus, message arbitration ensures only one ECU transmits at a time. | Faulty CAN Transceiver, Open in High or Low CAN Wire, Faulty ECU |
| Voltage Readings | CAN High: Ideally around 2.5V at rest (idle), rising to around 3.5V during data transmission. CAN Low: Ideally around 2.5V at rest (idle), falling to around 1.5V during data transmission. A U0013 code suggests CAN High voltage is significantly lower than 2.5V at rest. | Short to ground on CAN High, faulty ECU pulling down CAN High voltage. May require an oscilloscope to accurately observe voltage fluctuations during data transmission. |
| Termination Resistance | Should be approximately 60 ohms when measured across the CAN High and CAN Low wires with the ignition off. Incorrect resistance indicates a problem with the termination resistors or the CAN bus wiring. | Open termination resistor, shorted CAN High or CAN Low wire. |
| ECU Isolation | Disconnecting ECUs one at a time can help isolate the faulty unit pulling down the CAN High voltage. Monitor CAN High voltage after each disconnection to see if the voltage returns to normal. | Short to ground within an ECU, faulty CAN transceiver within an ECU. |
| Wiring Inspection | Thoroughly inspect the wiring harness for any signs of damage, such as cuts, abrasions, or rodent damage. Pay close attention to areas where the harness is routed near sharp edges or hot components. Check connectors for corrosion and loose terminals. | Environmental Damage, Mechanical Damage |
| Medium Speed CAN vs. High Speed CAN | Medium Speed CAN typically operates at 125 kbps, while High Speed CAN operates at 500 kbps. Medium Speed CAN is often used for body control functions, while High Speed CAN is used for powertrain and chassis control. | Incorrect CAN bus wiring, using the wrong type of CAN transceiver. |
| Diagnostic Tools | Scan tools capable of reading CAN bus data, multimeter for voltage and resistance measurements, oscilloscope for waveform analysis. | Lack of proper diagnostic equipment. |
| Impact on Vehicle Systems | ABS, Traction Control System, Power Steering, Body Control Module, Transmission Control Module, Instrument Cluster. | Failure of safety or comfort systems. |
| Common Connector Locations | OBD-II port, behind the dashboard, under the seats, in the engine compartment. | Difficulty accessing connectors for inspection and testing. |
| Related DTCs | U0001, U0002, U0010, U0011, U0012, U0100, U0101, U0121, U0155, U0401. These codes indicate general CAN bus communication issues or loss of communication with specific ECUs. | Multiple ECU failures, widespread wiring harness damage. |
| Repair Solutions | Repair damaged wiring, replace corroded connectors, replace faulty ECUs, replace faulty CAN transceiver, replace termination resistors. | Cost of replacement parts, labor costs. |
| Safety Precautions | Disconnect the battery before working on the electrical system. Be careful when probing wiring harnesses to avoid damaging the wires. Use proper grounding techniques to prevent static discharge. | Risk of electrical shock, damage to electronic components. |
Detailed Explanations:
Definition: The U0013 code specifically points to a "Low" voltage condition on the CAN High (+) wire within the Medium Speed CAN communication bus. This means the measured voltage on the CAN High wire is significantly lower than the expected resting voltage of approximately 2.5 volts.
Symptoms: Because the CAN bus is the backbone of communication between many of the vehicle's computers, a U0013 code can trigger a cascade of issues. You might experience warning lights illuminating on the dashboard (ABS, Traction Control, Check Engine), loss of functionality in systems like ABS or traction control, erratic gauge readings, or even a no-start condition. The specific symptoms depend on which ECUs are relying on the Medium Speed CAN bus for communication.
Troubleshooting: Diagnosing a U0013 code requires a systematic approach. Start by verifying the code with a scan tool and checking for any other related CAN bus DTCs. Then, visually inspect the wiring harness and connectors for damage or corrosion. Use a multimeter to measure the voltage on the CAN High and CAN Low wires, and check the termination resistance across the bus. If necessary, isolate ECUs one by one to identify the faulty unit. An oscilloscope can be helpful for analyzing the CAN bus waveforms.
CAN Bus Basics: The CAN bus uses a differential signaling system with two wires: CAN High and CAN Low. Data is transmitted by varying the voltage difference between these two wires. Termination resistors (typically 120 ohms each) are placed at each end of the bus to prevent signal reflections. ECUs communicate by transmitting messages on the bus, and a message arbitration system ensures that only one ECU transmits at a time.
Voltage Readings: In a healthy Medium Speed CAN bus, the CAN High wire should ideally be around 2.5V at rest (when no data is being transmitted). During data transmission, the CAN High voltage rises to approximately 3.5V. The CAN Low wire should also be around 2.5V at rest, and it falls to approximately 1.5V during data transmission. A U0013 code indicates that the CAN High voltage is significantly lower than 2.5V, suggesting a short to ground or a faulty ECU pulling down the voltage.
Termination Resistance: The termination resistance across the CAN High and CAN Low wires should be approximately 60 ohms with the ignition off. This is because there are typically two 120-ohm resistors in parallel, one at each end of the CAN bus. An incorrect resistance reading indicates a problem with the termination resistors or the CAN bus wiring. A higher resistance could indicate an open termination resistor, while a lower resistance could indicate a shorted CAN High or CAN Low wire.
ECU Isolation: If the CAN High voltage is consistently low, you can try disconnecting ECUs one at a time to see if the voltage returns to normal. This process helps isolate the faulty unit that is pulling down the CAN High voltage. After disconnecting each ECU, monitor the CAN High voltage to see if it returns to the expected value.
Wiring Inspection: A thorough visual inspection of the wiring harness is crucial for diagnosing CAN bus problems. Look for any signs of damage, such as cuts, abrasions, or rodent damage. Pay close attention to areas where the harness is routed near sharp edges or hot components. Also, check the connectors for corrosion and loose terminals. A "wiggle test," where you gently move the wiring harness while monitoring CAN bus data with a scan tool, can help identify intermittent wiring problems.
Medium Speed CAN vs. High Speed CAN: Vehicles often have multiple CAN buses operating at different speeds. Medium Speed CAN typically operates at 125 kbps and is often used for body control functions, such as power windows, door locks, and lighting. High Speed CAN operates at 500 kbps and is used for more critical functions, such as powertrain and chassis control. Mixing up the wiring between the two buses can cause communication problems and DTCs.
Diagnostic Tools: Diagnosing CAN bus problems requires specialized tools. A scan tool capable of reading CAN bus data is essential for retrieving DTCs and monitoring ECU communication. A multimeter is needed for measuring voltage and resistance. An oscilloscope can be helpful for analyzing CAN bus waveforms and identifying signal abnormalities.
Impact on Vehicle Systems: The Medium Speed CAN bus often connects ECUs related to ABS (Anti-lock Braking System), TCS (Traction Control System), Power Steering, Body Control Module (BCM), Transmission Control Module (TCM), and the Instrument Cluster. A failure of the CAN bus can disrupt communication between these ECUs, leading to malfunctions in these systems.
Common Connector Locations: CAN bus connectors can be found in various locations throughout the vehicle. Common locations include the OBD-II port, behind the dashboard, under the seats, and in the engine compartment. Identifying the correct connectors is essential for performing accurate voltage and resistance measurements.
Related DTCs: A U0013 code is often accompanied by other CAN bus related DTCs, such as U0001 (CAN Communication Bus), U0002 (CAN Communication Bus Performance), U0010 (Medium Speed CAN Communication Bus), U0011 (Medium Speed CAN Communication Bus (+)), U0012 (Medium Speed CAN Communication Bus (-)), U0100 (Lost Communication With ECM/PCM), U0101 (Lost Communication With TCM), U0121 (Lost Communication With ABS Control Module), U0155 (Lost Communication With Instrument Panel Cluster (IPC) Control Module), and U0401 (Invalid Data Received From ECM/PCM). These codes can provide additional clues about the nature of the communication problem.
Repair Solutions: Repairing a U0013 code often involves repairing damaged wiring, replacing corroded connectors, replacing faulty ECUs, replacing a faulty CAN transceiver (often integrated into an ECU), or replacing termination resistors. The specific repair depends on the root cause of the problem.
Safety Precautions: When working on the electrical system of a vehicle, it is important to disconnect the battery to prevent electrical shock. Be careful when probing wiring harnesses to avoid damaging the wires. Use proper grounding techniques to prevent static discharge, which can damage sensitive electronic components.
Frequently Asked Questions:
Question: What does a U0013 code mean?
Answer: It means the Medium Speed CAN communication bus (+) line has a low voltage, indicating a potential short to ground or a faulty ECU.
Question: Can I still drive my car with a U0013 code?
Answer: It depends on the severity of the issue. You might experience drivability problems or safety system malfunctions, so it's best to get it checked by a mechanic.
Question: What tools do I need to diagnose a U0013 code?
Answer: You'll need a scan tool, multimeter, and possibly an oscilloscope to accurately diagnose CAN bus problems.
Question: How do I check the CAN bus termination resistance?
Answer: Turn off the ignition and measure the resistance across the CAN High and CAN Low wires; it should be around 60 ohms.
Question: Where are the termination resistors located?
Answer: Typically, the termination resistors are located at each end of the CAN bus, often integrated into an ECU.
Conclusion:
The U0013 code indicates a serious issue with the Medium Speed CAN communication bus, potentially affecting various vehicle systems. A systematic approach to troubleshooting, including visual inspection, voltage measurements, and ECU isolation, is essential for identifying the root cause and implementing the appropriate repair. Consult a qualified technician if you are uncomfortable performing these diagnostic steps.