The code U0074: Control Module Communication Bus ‘B’ Off points out a fault in the communication bus ‘B’ of control modules in your vehicle. While it’s not an immediate concern, it can lead to communication problems between modules, potentially affecting various vehicle functions.
U0074 quick overview
| Meaning | U0074: Control Module Communication Bus ‘B’ Off |
| Is it serious? | This code is generally not serious on its own but can lead to communication problems between control modules, affecting various vehicle functions. |
| Possible causes | – Faulty control module (BCM, ECM, TCM, etc.) – Damaged or corroded wiring or connectors in the communication network. – Interference from aftermarket devices or modifications. – Low battery voltage or poor battery connections. – Software or programming issues in control modules. |
| How to diagnose? | Clear Diagnostic Codes: Start by clearing the U0074 code using a diagnostic tool, and observe if it returns. Inspect Wiring and Connectors: Thoroughly examine the wiring and connectors within the communication network for signs of damage, corrosion, or loose connections. Check Battery Voltage: Verify that the battery voltage falls within the correct range and confirm that the battery connections are clean and securely attached. Address Aftermarket Devices: If your vehicle has aftermarket devices or modifications, assess whether they may be causing interference with the communication network. Remove or isolate them as needed. Software and Programming: If the issue persists after performing the previous steps, consider checking and updating the software or programming of control modules. Consult a professional technician if necessary. |
U0074 Meaning
The U0074 code indicates a disruption in the communication network (bus ‘B’) that links various control modules in your vehicle, such as the Body Control Module (BCM), Engine Control Module (ECM), Transmission Control Module (TCM), and others. This communication is crucial for the proper functioning of different vehicle systems. When the code triggers, it means that the bus ‘B’ communication is temporarily disabled or disrupted, often due to issues like faulty modules, damaged wiring, or interference from external devices. While the vehicle may still run, it’s essential to address this problem to maintain seamless communication between modules and prevent potential complications.
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How to fix U0074 – learn from mechanics
I looked into U0074 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.
Service case #1: Chevrolet U0074
Vehicle Information: 2015 Chevrolet Silverado 1500 LT 5.3L
Issue + Repair History:
- SES light on with no other symptoms.
- Codes P2635, P018B, U0074, U18A2 present.
- Fuel pressure at 85.9 psi against desired 46.5 psi.
- Replaced fuel pressure sensor, fuel pump driver module, and fuel pump.
Other OBD2 Codes + Test Results:
- P2635: Fuel Pump Flow Performance
- P018B: Fuel Pressure Sensor Performance
- U0074: Control Module Communication Powertrain CAN Off
- U18A2: Lost Communication With Fuel Pump Module
Suggestions to Fix Issue:
- Check for TSBs addressing similar issues.
- Focus on U0074 code first due to its direct relation to P2635.
- Examine the 20 pin connector near the LF shock
Final Fix:
- Replaced and programmed the PCM.
- Fuel pressure and desired fuel pressure aligned correctly after PCM replacement.
- Problem resolved with the replacement of the PCM.
Service case #2: GMC U0074
Vehicle Information: 2012 GMC Terrain SLE 2.4L
Issue + Repair History:
- Vehicle displayed “service stabilitrak” message.
- Codes pulled: U0074, C0710 symptom 71, U0126, C0287, C0186, C0196.
- Steering angle sensor was replaced without resolving the issue.
Other OBD2 Codes + Test Results:
- Steering angle sensor reading “0”.
- Checked power and ground to steering angle sensor, found 12v and good ground.
Suggestions to Fix Issue:
- Check if the vehicle was in a wreck and if there’s communication with all modules.
- Inspect for data line problems.
- Consider checking the console for liquid damage near switches.
- Look for potential corrosion in connector X411 under the rear of the vehicle.
- Measure the circumference of all 4 tires to ensure they are the same.
- Review relevant TSBs for similar issues.
Final Fix:
- Used MDI with a laptop for further diagnostics.
- Found two software updates for the EBCM.
- Updated the EBCM controller.
- After update, drove the vehicle for 30+ miles in various conditions.
- Stabilitrak light and codes resolved following the EBCM software update.
Service case #3: Chevrolet U0074
Vehicle Information: 2012 Chevrolet Cruze LS 1.8L
Issue + Repair History:
- Service steering and traction control messages on dashboard.
- Car had been repaired for wires crushed by a chain.
- Replaced steering angle sensor, but issue persisted.
- Continuously showed 0 degrees on the steering sensor.
Other OBD2 Codes + Test Results:
- Codes: C0710, U0074, U0415.
- Checked connections and resistance, all normal.
Suggestions to Fix Issue:
- Check for blown fuses or fusible links.
- Ensure communication with EBCM.
- Follow diagnostics for U0074.
- Consider reprogramming with GM online SPS programming software.
Final Fix:
- Sent to a GM dealer for further diagnostics.
- Found that the Yaw sensor was bad.
- Replaced the Yaw sensor, clearing all codes and resolving issues.
- Discovered that the Snap-on scanner was causing misleading errors due to a wireless connector issue.
Service case #4: Cadillac U0074
Vehicle Information: 2010 Cadillac SRX Performance 3.0L
Issue + Repair History:
- “Service Stabilitrac” message and traction control light on.
- Numerous DTCs initially, all history, codes cleared.
- Replaced steering angle sensor (SAS) but communication issues persisted.
- Replaced differential module, which initially resolved the issue.
- Customer reported that Stabilitrac message returned intermittently.
Other OBD2 Codes + Test Results:
- Initial codes: C0710, U0074, U0125, U0126.
- After differential module replacement: U0073, U0074, U0125-72, U0125-74, U0126-72.
Suggestions to Fix Issue:
- Check for wire breaks in the wiring harness, particularly at X411.
- Consider using high-quality connection enhancers (CE1).
- Check terminal drag at rear differential control module.
- Investigate past issues related to rear drive system.
Final Fix:
- Found that the problem was temperature sensitive, often triggering on a cold start.
- Continued to investigate the blue pair high-speed data lines.
- Replaced female terminals at connector X202 due to loose connections.
- Replaced a loose terminal in the SAS connector.
- The issue was resolved after addressing terminal connection problems, highlighting the importance of checking everything, especially in vehicles that have been previously probed.
Service case #5: Buick U0074
Vehicle Information: 2011 Buick LaCrosse CXS 3.6L
Issue + Repair History:
- No start, no crank condition, began with a “click” noise when pressing the “Start” button.
- Attempted jump start and tapping on starter were unsuccessful.
- Vehicle eventually started after several hours.
- Current state: Vehicle starts and runs every time.
- Checked connections at battery, starter, and fuses; no problem found.
- OBD scan revealed several history communication and low voltage codes.
Other OBD2 Codes + Test Results:
- Codes: U0155, U0184, U0140, B1325, B1395, B1405, U0074, B097B, B1325, B1517.
- Battery load test: Passed – holds 11.5 volts at 200 amp draw.
- Starter draw test: Passed – 125 amp draw.
- Charging system test: 14.5 volts, 100+ amps.
- Voltage drop in ground and insulated circuits: 0.2 volts or less.
Suggestions to Fix Issue:
- Recheck battery load test with the correct limit of 10.0 volts.
- Clear all codes and observe which ones return.
- Check grounds from the engine to the body or battery.
- Investigate if there are issues with the fuse box or internal corrosion.
- Inquire about conditions when the problem occurs (temperature, auto lamp feature use, etc.).
Final Fix:
- Wired a test light to the “S” terminal on the starter for customer monitoring.
- After a few days, the problem recurred, indicating the starter was faulty.
- Replaced the starter, resolving the issue.
How to diagnose U0074
I made you an overview of solving this DTC, based on all the advice that was suggested by certified mechanics to address this issue in previously mentioned service cases.
| Advice | Diagnostic Step |
|---|---|
| Clear Codes and Monitor Returns | After repairing obvious issues, clear U0074 code and monitor if it returns. |
| Inspect Wiring Integrity | Check the wiring harness for damage, shorts, or breaks, especially in vulnerable areas. |
| Load Test Circuits | Perform a load test on the high-speed data lines (bus) to uncover intermittent issues. |
| Investigate External Factors | Inquire about external factors like weather conditions or recent vehicle repairs. |
| Utilize Proper Diagnostic Equipment | Use accurate diagnostic tools and software for troubleshooting and potential reprogramming. |
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I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko