Introduction:
The U0042 diagnostic trouble code (DTC) signifies a critical fault within a vehicle's Controller Area Network (CAN) bus system, specifically an "Open" circuit on the CAN bus B (-) line. The CAN bus serves as the central nervous system of modern vehicles, facilitating communication between various electronic control units (ECUs) or modules. Failure of this communication network can lead to a cascade of issues affecting vehicle performance, safety systems, and overall functionality.
Table: U0042 Vehicle Communication Bus B (-) Open
| Topic | Description | Potential Solutions |
|---|---|---|
| Definition | U0042 indicates an open circuit (break) in the CAN Bus B (-) wire. This prevents communication between ECUs on that specific bus. | N/A - This is the definition of the code. |
| CAN Bus Overview | The CAN bus is a serial communication network that allows different ECUs (e.g., engine control module, transmission control module, ABS module) to communicate with each other without complex, point-to-point wiring. It uses a differential signaling system, meaning data is transmitted using two wires (CAN High and CAN Low). | N/A - This is background information. |
| CAN Bus B | Modern vehicles often have multiple CAN buses (A, B, C, etc.) to manage the increasing complexity of electronic systems. CAN Bus B typically handles communication for less critical systems, such as body control, infotainment, or convenience features. However, its failure can still impact vehicle functionality and diagnostic capabilities. | N/A - This is about the specific bus. |
| Symptoms | Loss of communication between ECUs on CAN Bus B, malfunction of systems connected to CAN Bus B (e.g., power windows, door locks, infotainment system), illuminated warning lights (e.g., check engine light, ABS light, traction control light), stored DTCs in multiple modules, vehicle may exhibit drivability issues (depending on the importance of systems on CAN Bus B), and inability to diagnose modules on CAN Bus B with a scan tool. | Note: Not all symptoms may be present at once. |
| Causes | Open circuit in CAN Bus B (-) wire: This is the primary cause. The wire could be broken, cut, corroded, or disconnected. Faulty CAN Bus B (-) wire connector: A damaged or corroded connector can create an open circuit. Damaged ECU: A failing ECU connected to CAN Bus B could cause the bus to fail. Wiring harness damage: Physical damage to the wiring harness (e.g., rodent damage, chafing) can sever the CAN Bus B (-) wire. Short to ground: Although the code specifies an "Open," a severe short to ground on the CAN B (-) wire can sometimes mimic an open circuit condition to the diagnostic system. | Thorough visual inspection is required to pinpoint the exact cause. |
| Diagnostic Steps | Visual Inspection: Inspect the entire length of the CAN Bus B (-) wire for any signs of damage, corrosion, or disconnection. Pay close attention to connectors and areas where the wiring harness is routed near moving parts or heat sources. Continuity Test: Use a multimeter to perform a continuity test on the CAN Bus B (-) wire. Disconnect the wire from both ends and check for continuity between the two ends. No continuity indicates an open circuit. Voltage Test: With the ignition on, measure the voltage on the CAN Bus B (-) wire. It should typically be around 2.5 volts. A significantly different voltage may indicate a problem. Resistance Test: Measure the resistance between the CAN Bus B (-) wire and ground. A very low resistance (close to zero) may indicate a short to ground. ECU Isolation: Disconnect ECUs one at a time from CAN Bus B and retest. If the code clears after disconnecting a specific ECU, that ECU may be faulty. Wiring Diagram Review: Consult the vehicle's wiring diagram to identify all components connected to CAN Bus B and trace the wiring path. | Safety Note: Always disconnect the battery negative terminal before working on the electrical system. Consult the vehicle's service manual for specific testing procedures and voltage/resistance values. |
| Tools Required | Multimeter, wiring diagram, scan tool (for reading and clearing DTCs), wire stripper/crimper, soldering iron (if necessary for wire repairs), connector cleaner, visual inspection tools (flashlight, magnifying glass). | N/A |
| Common Mistakes | Assuming the ECU is faulty without thorough testing: ECUs are often replaced unnecessarily. Always perform a thorough diagnostic process before replacing an ECU. Ignoring wiring harness damage: Wiring harness damage is a common cause of CAN bus problems and should be carefully inspected. Not consulting a wiring diagram: A wiring diagram is essential for tracing the CAN bus wiring and identifying all connected components. Failing to properly terminate the CAN bus after repairs: CAN Bus requires proper termination to function. | N/A |
| Related DTCs | U0001, U0002, U0100 (Lost Communication with ECM/PCM "A"), U0101 (Lost Communication with TCM), and other U-codes related to lost communication with specific modules. These codes often appear in conjunction with U0042. | N/A |
| Severity | Moderate to High. While CAN Bus B often controls less critical systems, its failure can still impact vehicle functionality and diagnostic capabilities. It can also mask other underlying problems. | N/A |
| Termination Resistors | CAN Bus systems utilize termination resistors (typically 120 ohms) at each end of the bus to prevent signal reflection and ensure proper communication. A missing or faulty termination resistor can cause communication problems. The total resistance across the CAN High and CAN Low wires should be approximately 60 ohms when measured at any point on the bus with the system de-energized. | N/A |
| Repair Procedures | Wire Repair: If the CAN Bus B (-) wire is broken or damaged, repair it using proper splicing and soldering techniques. Ensure the repair is properly insulated to prevent corrosion and short circuits. Connector Repair/Replacement: If the connector is damaged or corroded, clean it with a connector cleaner or replace it entirely. ECU Replacement: Only replace an ECU after all other potential causes have been ruled out and the ECU has been confirmed to be faulty. Wiring Harness Repair: Repair any damage to the wiring harness, ensuring that all wires are properly insulated and protected. | Always use high-quality repair materials and follow proper repair procedures. |
Detailed Explanations:
Definition: The U0042 code signifies a break or open circuit specifically in the CAN Bus B (-) wire. This means the electrical circuit is incomplete, preventing the flow of communication signals.
CAN Bus Overview: The CAN bus is a sophisticated network that allows different modules in a vehicle to "talk" to each other. Instead of each module needing its own dedicated wiring to every other module, they all connect to a shared network. This simplifies wiring and reduces weight. The CAN bus uses a differential signaling system, meaning it transmits data as a difference in voltage between two wires: CAN High and CAN Low.
CAN Bus B: Vehicles often have multiple CAN buses to handle the increasing complexity of electronic systems. CAN Bus B is typically used for systems that are less critical to the immediate operation of the vehicle, such as body control functions (power windows, door locks, etc.), infotainment systems, and convenience features. However, issues with CAN Bus B can still impact the overall functionality of the vehicle and prevent proper diagnosis of other systems.
Symptoms: The symptoms of a U0042 code can vary depending on which systems are connected to CAN Bus B in the specific vehicle. Common symptoms include: loss of communication between ECUs on CAN Bus B, malfunctions of systems connected to CAN Bus B (e.g., power windows not working, door locks malfunctioning, infotainment system errors), illuminated warning lights (check engine light, ABS light, traction control light), stored DTCs in multiple modules because they can't communicate, drivability issues (if systems on CAN Bus B are related to engine or transmission control), and an inability to diagnose modules on CAN Bus B using a scan tool.
Causes: The most common cause is an open circuit in the CAN Bus B (-) wire. This means the wire is physically broken, cut, or disconnected somewhere along its length. A faulty CAN Bus B (-) wire connector can also cause an open circuit if the connector is damaged, corroded, or not properly connected. A damaged ECU connected to CAN Bus B could potentially cause the bus to fail, although this is less common than wiring problems. Wiring harness damage from rodents, chafing, or other physical damage can sever the CAN Bus B (-) wire. Finally, in some rare cases, a severe short to ground on the CAN B (-) wire can mimic an open circuit to the diagnostic system.
Diagnostic Steps: Start with a visual inspection of the entire CAN Bus B (-) wire, looking for any signs of damage, corrosion, or disconnection. Pay close attention to connectors and areas where the wiring harness is routed near moving parts or heat sources. Next, perform a continuity test on the CAN Bus B (-) wire using a multimeter. Disconnect the wire from both ends and check for continuity between the two ends. No continuity indicates an open circuit. With the ignition on, perform a voltage test on the CAN Bus B (-) wire. It should typically be around 2.5 volts. A significantly different voltage may indicate a problem. Then measure the resistance between the CAN Bus B (-) wire and ground. A very low resistance (close to zero) may indicate a short to ground. If you suspect a faulty ECU, try ECU isolation by disconnecting ECUs one at a time from CAN Bus B and retesting. If the code clears after disconnecting a specific ECU, that ECU may be faulty. Finally, consult the vehicle's wiring diagram to identify all components connected to CAN Bus B and trace the wiring path.
Tools Required: You will need a multimeter to perform continuity, voltage, and resistance tests. A wiring diagram is essential for tracing the CAN bus wiring. A scan tool is needed for reading and clearing DTCs. You'll also need wire strippers/crimpers, a soldering iron (if necessary for wire repairs), connector cleaner, and visual inspection tools (flashlight, magnifying glass).
Common Mistakes: A common mistake is assuming the ECU is faulty without thorough testing. ECUs are expensive, and it's important to rule out other potential causes first. Ignoring wiring harness damage is another common mistake. Wiring harness damage is a frequent cause of CAN bus problems and should be carefully inspected. Not consulting a wiring diagram can make it difficult to trace the CAN bus wiring and identify all connected components. Failing to properly terminate the CAN bus after repairs can also cause problems. CAN Bus requires proper termination to function correctly.
Related DTCs: Related DTCs often include U0001, U0002, U0100 (Lost Communication with ECM/PCM "A"), U0101 (Lost Communication with TCM), and other U-codes related to lost communication with specific modules. These codes often appear in conjunction with U0042 because the failure of CAN Bus B can prevent communication with other modules.
Severity: The severity of a U0042 code is moderate to high. While CAN Bus B often controls less critical systems, its failure can still impact vehicle functionality and diagnostic capabilities. It can also mask other underlying problems.
Termination Resistors: CAN Bus systems utilize termination resistors (typically 120 ohms) at each end of the bus to prevent signal reflection and ensure proper communication. A missing or faulty termination resistor can cause communication problems. The total resistance across the CAN High and CAN Low wires should be approximately 60 ohms when measured at any point on the bus with the system de-energized.
Repair Procedures: If the CAN Bus B (-) wire is broken or damaged, repair it using proper splicing and soldering techniques. Ensure the repair is properly insulated to prevent corrosion and short circuits. If the connector is damaged or corroded, clean it with a connector cleaner or replace it entirely. Only replace an ECU after all other potential causes have been ruled out and the ECU has been confirmed to be faulty. Repair any damage to the wiring harness, ensuring that all wires are properly insulated and protected.
Frequently Asked Questions:
Question: What does the U0042 code mean?
Answer: The U0042 code indicates an open circuit (break) in the CAN Bus B (-) wire, preventing communication on that specific bus. This disrupts communication between ECUs connected to that bus.
Question: What are the common symptoms of a U0042 code?
Answer: Common symptoms include malfunctions of systems connected to CAN Bus B (like power windows or infotainment), illuminated warning lights, and an inability to diagnose modules on CAN Bus B.
Question: What is the most likely cause of the U0042 code?
Answer: The most likely cause is a broken, cut, or disconnected CAN Bus B (-) wire or a faulty connector on that wire.
Question: Can I fix the U0042 code myself?
Answer: If you have experience with automotive electrical systems and are comfortable using a multimeter and wiring diagrams, you may be able to diagnose and repair the problem yourself. However, if you are not comfortable with these tasks, it is best to take the vehicle to a qualified mechanic.
Question: How important is it to fix a U0042 code?
Answer: While CAN Bus B often controls less critical systems, its failure can still impact vehicle functionality and diagnostic capabilities. It's important to address the issue to restore full vehicle functionality and prevent potential problems down the road.
Conclusion:
The U0042 code signifies a break in the CAN Bus B (-) wire, disrupting communication between ECUs. Thorough diagnosis, starting with a visual inspection and followed by electrical testing, is crucial for pinpointing the cause and implementing the appropriate repair.