|Code||Fault Location||Probable Cause|
|U0155|| Data bus: instrumentation control module - no communication |
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Table of Contents
- What Does Code U0155 Mean?
- Where is the U0155 sensor located?
- What are the common causes of code U0155?
- Get Help with U0155
What Does Code U0155 Mean?
OBD II fault code U0155 is a generic code that is defined as “Lost Communication With Instrument Panel Cluster (IPC) Control Module”, and is set when the PCM (Powertrain Control Module) detects a loss of communication between itself and the instrument panel cluster control module, or between the instrument panel cluster control module and one or more other control modules on the CAN (Controller Area Network) serial bus communication system.
Most modern vehicles have upwards of 20 microcontrollers or control modules, and many have more, all of which are concerned or involved with controlling, monitoring, or regulating one or more functions that ensure the smooth and efficient operation of all the systems that make modern vehicles work.
One such control module is the instrument cluster that conveys vitally important information about the overall condition and efficiency of major operational systems to the driver. Examples of this information would be the vehicles’ speed, temperature, and pressures of various fluids like engine oil and coolant, and warning lights that indicate the status of important systems like safety systems, among others.
To integrate all of these systems and their control modules, modern vehicles use a complex web of harnesses, known as the CAN (Controller Area Network) bus system. This system interconnects all the control modules in the application through thousands of circuits to allow systems, subsystems, and a plethora of sensors to communicate both with the PCM and with each other according to strictly maintained communication protocols. However, while all control modules transmit and receive information that includes identifying the communicating control modules, the information from and to some control modules are deemed more important than information to and from other modules.
One example of this is information/communication between the PCM and the instrument cluster. If this communication link is broken for whatever reason, many other control modules cannot communicate with the PCM(or with each other) via the instrument cluster, which in practice, means that the PCM cannot oversee and/or monitor the efficient operation of some critically important control modules.
One practical example of this should suffice; since effective control of an automatic transmission depends on (among other factors) the vehicles’ road speed, engine speed, and throttle position, the PCM cannot communicate with the transmission control module if the communication link between the PCM and the instrument cluster is broken. In this case, the vehicles’ road speed is relayed to the PCM via the instrument cluster, which means that if no communication is possible, the transmission cannot be controlled effectively, if at all.
Thus, if the PCM detects a break in communication between itself and the instrument cluster, it will set code U0155, and possibly a warning light as well. Note that depending on where in the CAN bus system the failure occurred, the PCM might initiate a fail-safe or limp mode- particularly if transmission and throttle control is affected.
Where is the U0155 sensor located?
The image above shows the instrument cluster panel from a Ford Focus application. Note the black square just to right of centre; this is in most cases a CPU (Central Processing Unit) that acts as a sort of “clearing house” for all the signals from control modules that enter and leave the instrument cluster via the CAN bus system.
Also note that while instrument clusters are sometimes repairable by suitably qualified persons, the repaired instrument cluster and the PCM almost always needs to be reprogrammed with the manufacturer’s anti-theft protocols at the same time. This is especially true of Ford applications, which may not start if the correct reprogramming procedures are not followed.
What are the common causes of code U0155?
Some common causes of code U0155 could include the following-
- Open circuit in the CAN bus positive circuit
- Open circuit in the CAN bus negative circuit
- Short circuit in either or both CAN bus circuits
- One or more failed or failing control modules other than the PCM
- Failed or failing PCM, but note that while this is not altogether impossible where UXXXX codes are concerned, it is nevertheless an exceedingly rare occurrence
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