U0074 Communication bus control module B is off

U0074 Communication bus control module B is off

OBD-II DTC Datasheet

Control module communication bus "B" Off.

What does this mean?

This communication diagnostic trouble code usually applies to most domestic and imported fuel injection engines manufactured since 2004. These manufacturers include, but are not limited to, Acura, Buick, Chevrolet, Cadillac, Ford, GMC, and Honda.

This code is associated with the communication circuit between the control modules on the vehicle. This communication chain is most often referred to as the Controller Area Network bus communication or, more simply, the CAN bus. Without this CAN bus, control modules cannot communicate and your scan tool may not be able to communicate with the vehicle, depending on which circuit is involved.

Troubleshooting steps may vary depending on the manufacturer, the type of communication system, the color of the wires, and the number of wires in the communication system. U0074 refers to bus "B" while U0073 refers to bus "A".

symptoms

Symptoms of a U0074 engine code may include:

• Malfunction Indicator Lamp (MIL) illuminated
• Lack of power
• Poor fuel economy
• The indicator of all instrument clusters is "on"
• Possibly no cranking, no starting condition

reasons

Possible reasons for setting this code:

• Open in the CAN + bus circuit "B"
• Open in the bus CAN "B" - electrical circuit
• Short circuit to power in any CAN-bus circuit "B"
• Short circuit on ground in any CAN-bus circuit "B"
• Rarely - the control module is faulty

Diagnostic and repair procedures

A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.

Check first if you can access the trouble codes, and if so, notice if there are other diagnostic trouble codes. If any of these are related to module communication, first diagnose them. It is known that a misdiagnosis occurs if a technician diagnoses this code before any other system codes associated with module communication are thoroughly diagnosed.

Then find all the bus connections on your particular vehicle. Once detected, visually inspect the connectors and wiring. Look for scuffs, scuffs, exposed wires, burn marks, or molten plastic. Disconnect the connectors and carefully inspect the terminals (metal parts) inside the connectors. See if they look rusty, burnt, or possibly green compared to the usual metallic color you're probably used to seeing. If terminal cleaning is required, you can purchase electrical contact cleaner at any parts store. If this is not possible, find 91% rubbing alcohol and a light plastic bristle brush to clean them. Then let them air dry, take a dielectric silicone compound (the same material they use for bulb holders and spark plug wires) and place where the terminals make contact.

If your scan tool can now communicate, or if there were any DTCs related to module communication, clear the DTCs from memory and see if the code returns. If this is not the case, then there is most likely a connection problem.

If communication is not possible or you were unable to clear module communication related trouble codes, the only thing you can do is disable one control module at a time and see if the scan tool is communicating or if the codes are cleared. Disconnect the negative battery cable before disconnecting the connector on this control module. Once disconnected, disconnect the connector(s) on the control module, reconnect the battery cable and repeat the test. If there is communication now or the codes are cleared, then this module/connection is faulty.

If communication is not possible or you have not been able to clear module communication related trouble codes, the only thing that can be done is to seek the assistance of a trained automotive diagnostician.

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