U1102 – Loss of Serial Communications for Class 2 Devices

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By Reinier (Contact Me)
Last Updated 2020-08-13
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
U1102 SCP (J1850) Invalid or Missing Data for Primary Id
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Table of Contents

  1. What Does Code U1102 Mean?
  2. Where is the U1102 sensor located?
  3. What are the common causes of code U1102?
  4. What are the symptoms of code U1102?
  5. Get Help with U1102

What Does Code U1102 Mean?

OBD II fault code U1102 is a manufacturer-specific trouble code that is defined by carmaker GM as “Loss of Serial Communications for Class 2 Devices”, and is set when the PCM (Powertrain Control Module) detects a break or interruption in the communication system(s) between Class 2 devices.

The term “Class 2 devices” is a bit misleading, since at first glance it seems to refer to some devices (control modules in GM-speak) that exist separately from other classes of devices. In practice, Class 2 refers to a serial bus communications system that was in use on GM products between about 1980/81 and about 2000 or slightly later. To comply with Federal emissions regulations, this system used a sort communications protocol known as UART (Universal Asynchronous Receiver/Transmitter) that essentially, used only two wires to connect all control modules.

As a practical matter, this arrangement meant that all control modules could communicate with all other control modules all the time, as opposed to modern systems that use a system of arbitration to decide which types of communication gets precedence over any other type of communication.

Nonetheless, UART used the SAE J1850 VPW standard/protocol for diagnostic functions, and this largely depended on a UART requirement that all control modules transmit state-of-health messages with all other control modules every few seconds, regardless of any other data transmissions that may or may not be taking place at that moment. In this way, it was possible for J1850 VPW standard to identify faults and failures between control modules simply by noting which control module had stopped communicating, or was communication erratically. This is an admittedly gross oversimplification of the diagnostic process, but it does illustrate the point that Class 2 systems relied less on sensor outputs to diagnose faults than modern systems do.

While this system worked reasonably well for the relatively uncomplicated vehicles of the time, rapid advances in technology, and especially emissions and safety-related technologies began to outpace the capabilities of UART-based systems in terms of both speed of transmission, and the need to use different protocols for different functions on modern vehicles. As a result, UART systems became obsolete about ten years after they were first introduced in 1995/6.

Thus, since there are relatively few UART-based GM vehicles in use today, it should be noted that UART-specific diagnostic equipment has become equally rare. Nevertheless, when a PCM in a Class 2 UART-based system detected a break in communications between two or more control modules, it set code U1102 as a result.

Where is the U1102 sensor located?

This image shows the location (circled) of the PCM and other major control modules on a circa 1996 C4 Corvette, but note that accessing this location requires substantial disassembly of the dashboard. The location of the PCM on other GM products from the same period are for the most part also located under or behind the dashboard, but other typical locations include placement under front seats, behind kick panels, or in a few cases, in the trunk. Consult reliable service information for more detailed information on PCM locations for affected vehicles.

What are the common causes of code U1102?

Common causes of code U1102 on GM applications could include one or more of the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
  • One or more defective and/or failing control modules, including the PCM
  • Abnormal system voltages
  • Incorrect jump start procedures
  • Charging system failures and/or defects
  • Defective or failing battery

What are the symptoms of code U1102?

Common symptoms are largely dependent on the specific model and nature of the problem, but could include one or more of the following-

  • Multiple stored trouble codes and several illuminated warning lights
  • Vehicle may be immobilized
  • No-start condition may be present
  • Instruments may be frozen in the position they were in when the fault occurred

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