U011A Lost communication with exhaust gas sensor module

OBD-II DTC Datasheet

Lost Communication With Exhaust Gas Sensor Module

What does this mean?

This is a generic communications system diagnostic trouble code that applies to most makes and models of OBD-II vehicles.

This code means that the Exhaust Gas Sensor Module (EGSM) 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.

Modules communicate with each other over a network, just like the network you have at home or work. Car manufacturers use several networked systems. Prior to 2004, the most common (non-exhaustive) inter-module communication systems were the serial communication interface, or SCI; SAE J1850 or PCI bus; and Chrysler Collision Detection, or CCD. The most common system used after 2004 is known as Controller Area Network communication, or simply the CAN bus (also used until 2004 on a small segment of vehicles). Without this CAN bus, control modules cannot communicate and your scan tool may or may not receive information from the vehicle, depending on which circuit is affected.

The exhaust gas sensor module (EGSM) is usually located under the hood, either on the fender or on the bulkhead. It receives input from various sensors, some of which are directly connected to it, and most are sent over a bus communication system from the powertrain control module (PCM). These inputs allow the module to inform the PCM of the oxygen content inside the exhaust to ensure complete combustion and reduce exhaust emissions.

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.

Code severity and symptoms

The severity in this case is not serious because PCM has a backup strategy in case it loses communication with the EGSM.

Symptoms of a U011A code may include:

EGSM does not turn on / does not work

reasons

Usually the reason for installing this code is:

Open on CAN bus + or - circuit
Short to ground or ground in any CAN bus circuit
No power or ground to the EGSM
Rarely - the control module is faulty

Diagnostic and repair procedures

A good place to start with ALL electrical diagnostics is to check the Technical Service Bulletins (TSB) for your vehicle. The problem you are facing may be known to others in the field. A known fix may have been released by the manufacturer and can save you time and money during diagnostics.

It is assumed that a code reader is available to you at this point, as you may have been able to access the codes up until now. See if there were any other DTCs related to bus communication or battery / ignition. If so, you should diagnose them first, as misdiagnosis is known to occur if you diagnose the U011A code before any of the underlying codes are thoroughly diagnosed and corrected.

If the only code you get from other modules is U011A, try accessing EGSM. If you can access codes from EGSM then code U011A is either intermittent or memory code. If the EGSM cannot be accessed, then code U011A set by other modules is active and the problem already exists.

The most common failure is a circuit failure that cuts power or ground to the exhaust gas sensor module.

Check all fuses supplying the EGSM on this vehicle. Check all the grounds for an EGSM. 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 any modules that set the code in memory and see if you can now communicate with the EGSM. If communication with the EGSM is restored, the problem is most likely a fuse / connection issue.

If the code returns or communication with the module still cannot be established, locate the CAN bus communication connections on your vehicle, most importantly the EGSM connector, which is usually found under the hood on the fenders or on the bulkhead. Disconnect the negative battery cable before disconnecting the connector on the EGSM. 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 plugging the connectors back into the EGSM. You will need access to a digital volt-ohmmeter (DVOM). Make sure the EGSM is powered and grounded. Access the wiring diagram and determine where the main power and ground supplies go into the EGSM. Reconnect the battery before proceeding with the EGSM still disconnected. Connect the red wire of your voltmeter to each B + (battery voltage) power source going into the EGSM connector, and the black wire of your voltmeter to a good ground (if unsure, the negative pole of the battery always works). You should see the battery voltage reading. Make sure you have a good reason. Connect the red lead from the voltmeter to battery positive (B +) and the black lead to each ground. Once again, you should see the battery voltage every time you plug it in. If not, troubleshoot 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. Other manufacturers show CAN C- at about 5V and an oscillating key with the engine off. Check your manufacturer's specifications.

If all tests pass and communication is still not possible, or you were unable to reset DTC U011A, the only thing that can be done is to seek help from a trained automotive diagnostician as this will indicate an EGSM failure. Most of these EGSMs must be programmed or calibrated in order to properly install the vehicle.

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