U0151 Lost Communication With Restraint Control Module (RCM)

U0151 Lost Communication With Restraint Control Module (RCM)

OBD-II DTC Datasheet

Lost Communication With Restraint Control Module (RCM)

What does this mean?

This is a generic communication system diagnostic trouble code that applies to most makes and models of vehicles.

This code means that the Restriction Control Module (RCM) and other control modules on the vehicle are not communicating with each other. The circuitry most commonly used for communication is known as Controller Area Bus communication, or simply the CAN bus.

Without this CAN bus, control modules cannot communicate and your scan tool may not receive information from the vehicle, depending on which circuit is involved.

The RCM is usually located behind the dashboard, usually in the center of the vehicle. It accepts input data from various sensors, some of which are directly connected to it, and some are sent over a bus communication system. The most important inputs of these sensors are the collision or collision sensors. This input allows the module to determine when a collision has occurred or when the vehicle is simply slowing down quickly. The difference between the two is that the RCM can take no action, can activate the seat belt pretensioners, or activate the pretensioners and passive restraints / airbags.

Troubleshooting steps may vary depending on the manufacturer, the type of communication system, the number of wires, and the colors of the wires in the communication system.

Severity and symptoms

The severity in this case is always serious due to the safety issues that arise in a non-functional restraint control system. Safety is ANYONE's concern when you service these systems because they can continue to operate even when the warning lights are on. ALWAYS treat these systems as if they could work at any time.

Symptoms of a U0151 engine code may include:

• Airbag light is on or flashing

reasons

Usually the reason for installing this code is:

• Open in CAN + bus circuit
• Open in the CAN bus - electrical circuit
• Short circuit to power in any CAN bus circuit
• Short to ground in any CAN bus circuit
• 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.

First, look for other DTCs. If any of these are related to the tire, vehicle identification number (VIN), or battery / ignition, diagnose them first. Misdiagnosis is known to occur if you diagnose the U0151 code before any of the major codes are thoroughly diagnosed and rejected.

If your scan tool can access trouble codes and the only code you are getting from other modules is U0151, try accessing the RCM. If you can access the codes from the RCM, then code U0151 is either an intermittent or memory code. If the RCM cannot be accessed, then code U0151 that other modules set is active and the problem already exists.

The most common failure is a loss of power or ground to the RCM.

Check all fuses supplying the RCM on this vehicle. Check all the reasons for RCM. Locate ground anchorage points on the vehicle and make sure these connections are clean and secure. If necessary, remove them, take a small wire bristle brush and baking soda / water solution and clean each one, both the connector and the place where it connects.

If any repairs have been made, clear the DTCs from memory and see if U0151 returns or you can contact the RCM. If no code is returned or communication is restored, the problem is most likely a fuse / connection issue.

BEFORE DISCONNECTING THE CONNECTORS ON THE RCM, MAKE SURE THE SYSTEM IS TURNED OFF ACCORDING TO THE MANUFACTURER'S PROCEDURES! IF NO, POSSIBLE VEHICLE DAMAGE OR PERSONAL INJURY DUE TO UNINTENDED DEPLOYMENT OF THE AIRBAG. ALSO, THE AIRBAGS ARE DISCONNECTED DURING THE FOLLOWING TESTS AS A FINAL SAFETY MEASURE!

If the code returns, look for the CAN bus connections on your specific vehicle, especially the RCM connector. Disconnect the negative battery cable before disconnecting the connector on the RCM. Once detected, visually inspect the connectors and wiring. Look for scratches, 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 burnt or have a green tint indicating corrosion. If you need to clean the terminals, use an electrical contact cleaner and a plastic bristle brush. Allow to dry and apply electrical grease where the terminals touch.

Perform these few voltage checks before connecting the connectors back to the RCM. You will need access to a digital volt/ohmmeter (DVOM). Make sure the RCM has power and ground. Access the wiring diagram and determine where the main power and ground supplies enter the RCM. Reconnect the battery before continuing with the RCM still disabled. Connect the red lead of your voltmeter to each B+ (battery voltage) power supply plugged into the RCM connector, and the black lead of your voltmeter to a good ground (if unsure, battery negative always works). You should see the battery voltage reading. Make sure you have a good reason. Connect the red lead of the voltmeter to the battery positive (B+) and the black lead to each ground circuit. Once again, you should see the battery voltage every time you connect. If not, repair the power or ground circuit.

Before proceeding, check your wiring diagram and see if you have one or both of these different communication circuits in the RCM; Carry out the checks related to the circuits in your vehicle.

Then check the two communication circuits. Locate CAN C+ (or HSCAN+) and CAN C- (or HSCAN - circuit). With the black wire of the voltmeter connected to a good ground, connect the red wire to CAN C+. With the key on and engine off, you should see about 2.6 volts with little fluctuation. Then connect the red wire of the voltmeter to the CAN C- circuit. You should see about 2.4 volts with little fluctuation.

Then check the other two communication circuits. Locate CAN B+ (or MSCAN + circuit) and CAN B- (or MSCAN - circuit). With the black wire of the voltmeter connected to a good ground, connect the red wire to CAN B+. With the key on and the engine off, you should see a voltage of about 0.5 volts with little fluctuation. Then connect the red lead of a voltmeter to the CAN B circuit. You should see about 4.4 volts with slight fluctuations.

If all tests pass and communication is still not possible, or you were unable to reset DTC U0151, the only thing to do is to seek help from a trained automotive diagnostician as this will point to a bad RCM. Most of these RCMs need to be programmed or calibrated for the vehicle in order to install them correctly.

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