Damaged, corroded, or loose wiring and connector pins on the CAN bus circuit feeding the rocker arm control module A.; Failed rocker arm control module A internal electronics or power supply, causing it to drop off the network.; Blown fuse or poor ground connection that removes power from the rocker arm control module A.; CAN bus network fault such as a shorted or open data line that isolates the module from the rest of the vehicle.; Software or calibration issue in the ECM or rocker arm control module that interrupts message timing.; Aftermarket accessories or poor previous repairs tapping into the network wiring near the module.

DTC codes / U01xx · Lost communication with a control module / U011B

U011BLost Communication With Rocker Arm Control Module A

Q: What does code U011B mean?

Lost Communication With Rocker Arm Control Module A. U011B is a communication fault where the engine control module (ECM) or another primary module has lost the controller area network (CAN) signal from the rocker arm control module A. The computer detects this when expected periodic messages from that module stop arriving on the network bus, not necessarily when the rocker arm hardware itself has failed.

U011B is a communication fault where the engine control module (ECM) or another primary module has lost the controller area network (CAN) signal from the rocker arm control module A. The computer detects this when expected periodic messages from that module stop arriving on the network bus, not necessarily when the rocker arm hardware itself has failed.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaLost communication with a control module

Symptoms you may notice

Illuminated check engine light or service vehicle soon message with no other obvious change in driving behavior.
Rough idle or misfire-like shudder if the rocker arm control module manages variable valve lift or cylinder deactivation.
Reduced engine power or a limp-home mode triggered by the ECM when communication is lost.
Intermittent stalling or hesitation that comes and goes with temperature or vibration.
Often no symptoms besides the warning light when the fault is a brief network glitch or a stored history code.

Common causes, most likely first

Damaged, corroded, or loose wiring and connector pins on the CAN bus circuit feeding the rocker arm control module A.
Failed rocker arm control module A internal electronics or power supply, causing it to drop off the network.
Blown fuse or poor ground connection that removes power from the rocker arm control module A.
CAN bus network fault such as a shorted or open data line that isolates the module from the rest of the vehicle.
Software or calibration issue in the ECM or rocker arm control module that interrupts message timing.
Aftermarket accessories or poor previous repairs tapping into the network wiring near the module.

How to diagnose and fix it

Scan all modules for communication codes and record whether U011B is current, history, or pending before clearing anything.
Inspect the rocker arm control module A connectors for corrosion, pushed pins, and loose terminals, then check the module's power and ground with a multimeter.
Check the fuse and ground circuit that supply the rocker arm control module A, repairing any open or high-resistance connection.
Measure CAN bus resistance and voltage at the module connector to confirm the data lines are intact and not shorted.
Perform a wiggle test on the harness while watching live network data to catch intermittent wiring faults.
If wiring and power are good, substitute a known-good rocker arm control module A or reflash the module and ECM with the latest calibration.
Clear codes and complete a drive cycle to verify the communication fault does not return.

What the network code really tells you

U011B is a communication code, so the rocker arm control module A may be perfectly functional mechanically while a single bad ground or chafed wire sets the code.
A weak or failing battery can cause multiple U-codes at once, so check battery health and charging voltage before condemning any module.
Aftermarket remote starters, alarms, or trailer wiring tapped into the CAN bus are a common hidden cause of lost communication with a specific module.
The ECM may store U011B as history after a brief voltage dip during cranking, and it will not return if the underlying power supply is repaired.
Do not replace the rocker arm control module A on the code alone; confirm the module actually drops off the network with a scan tool live data session.

Can I keep driving with U011B?

The vehicle may drive normally, but a lost rocker arm control module can affect valve timing or cylinder deactivation and cause drivability problems or limp mode. Avoid heavy acceleration and long trips until the communication fault is diagnosed, and do not clear the code repeatedly without checking the wiring and power supply.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.