What does it 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.
Symptoms
- Multiple warning lamps on and several modules reporting communication faults
- Scan tool cannot connect to some or all modules
- Gauges reading incorrectly or not at all
- Engine may run poorly, refuse to start or stall
- Body electrics such as lighting and locking behaving unpredictably
- Fault may come and go with vibration, temperature or humidity
Likely causes
- Open circuit or high resistance in the single serial data wire linking the modules
- Serial data line shorted to ground or to battery voltage, stopping traffic for every module on it
- One failed module holding the data line low and blocking communication for the whole network
- Corroded splice pack or connector, a very common fault on older harnesses
- Poor module grounds, since this network depends heavily on a clean common ground reference
- Water damage in a footwell or under-dash connector
- Aftermarket accessory spliced into the serial data line and loading it down
How to fix it
- Establish which modules still respond to the scan tool, as that alone narrows the affected section of the network
- Check the serial data wire for continuity between modules and for shorts to ground and to battery voltage
- Locate and inspect the splice packs on the data circuit, which are a frequent source of corrosion on older vehicles
- Disconnect modules one at a time and retest; if communication is restored when one module is unplugged, that module is loading the line
- Clean and retighten all module ground points, then verify them with voltage drop tests
- Inspect under-dash and footwell connectors for water damage and corroded terminals
- Remove any aftermarket device wired into the data line and retest
- Repair the wiring or replace the module that is loading the network, then confirm every module communicates again