U0121 Lost Communication With Anti-lock Braking System (ABS) Control Module

DTC U0121 - OBD-II Datasheet

Lost Communication With Anti-lock Braking System (ABS) Control Module

What does error U0121 mean?

This is a generic communication system diagnostic trouble code that applies to most makes and models of vehicles. This may include but is not limited to Mazda, Chevrolet, Dodge, VW, Ford, Jeep, GMC, etc.

This code is associated with the communication circuit between the anti-lock braking system (ABS) control module and other control modules on the vehicle.

This communication chain is most commonly referred to as the Controller Area Network 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 number of wires, and the colors of the wires in the communication system.

Symptoms

Symptoms of a U0121 engine code may include:

• Malfunction Indicator Lamp (MIL) illuminated
• ABS indicator is on
• TRAC indicator is on (depending on the manufacturer)
• ESP / ESC indicator is on (depending on the manufacturer)

Causes of error U0121

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.

If your scan tool can access trouble codes and the only code you are pulling from other modules is U0121, try accessing the ABS module. If you can access the codes from the ABS module, then code U0121 is either an intermittent or memory code. If the codes for the ABS module cannot be accessed, then code U0121 set by other modules is active and the problem already exists.

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

Check all fuses supplying the ABS module on this vehicle. Check all grounds of the ABS module. Locate grounding attachment 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 U0121 returns or you can contact the ABS module. If no code is returned or communication is restored, the problem is most likely a fuse / connection issue.

If the code returns, look for the CAN C bus connections on your specific vehicle, especially the ABS module connector. Disconnect the negative battery cable before disconnecting the connector on the ABS control module. 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 at any parts store. Allow to dry and apply dielectric silicone grease where the terminals touch.

Perform these few voltage checks before reconnecting the connectors to the ABS module. You will need access to a digital volt/ohmmeter (DVOM). Make sure you have power and ground on the ABS module. Access the wiring diagram and determine where the main power and ground supplies enter the ABS module. Reconnect the battery before proceeding with the ABS module disabled. Connect the red lead of a voltmeter to each B+ (battery voltage) power supply plugged into the ABS module connector, and the black lead of the voltmeter to a good ground (if unsure, battery negative always works). You 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.

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.

If all tests pass and communication is still not possible or you were unable to reset DTC U0121, the only thing you can do is seek help from a trained automotive diagnostician as this will indicate a faulty ABS module. Most of these ABS modules need to be programmed or calibrated for the vehicle in order to install them correctly.

Common Mistakes

The following are some of the most common mistakes a technician can make when diagnosing code U0121:

• No freeze frame data check to determine the conditions under which the DTC was set.
• Do not check the vehicle documents to make sure the code is correct and not another code.
• Use of diagnostic equipment that misinterprets DTCs and reports scan results to them.
• Not all tests or paging tests required to identify faulty components are not run. It may be easy to identify one or two faulty components, but all tests must be performed in order to make an accurate diagnosis.
• Before replacing any components or parts, always check their compatibility with the vehicle.

How serious is this?

Code U0121 is considered serious. If a vehicle shows symptoms, it should be diagnosed as soon as possible to avoid further damage and the possibility of an accident.

What repair can fix the code?

Below are solutions that can fix this problem:

• First write down any trouble codes that may be present.
• Check the wiring and ABS brake control module location using the service manual. Inspect wiring for obvious signs of damage such as wear, corrosion, or burns. Repair any damaged areas.
• Check the resistance, reference voltage, continuity, and ground signal of the system harness as instructed in the service manual. If you find a value out of range, take appropriate corrective action.

U0121 DIAG/FIXED Chevy "NO COMMUNICATION"

Related codes

Code U0121 is associated with and may be accompanied by the following codes:

P0021 , P0117 , P0220, P0732, P0457 , P0332 , U0401 , P2005 , P0358 , P0033 , P0868 , P0735

Need more help with code u0121?

If you still need help with DTC U0121, post a question in the comments below this article.

NOTE. This information is provided for informational purposes only. It is not intended to be used as a repair recommendation and we are not responsible for any action you take on any vehicle. All information on this site is protected by copyright.