U0091 is a network communication fault indicating that the 'G' bus (a specific vehicle communication network) has been detected as shorted. This code is set when the module monitoring the bus (often the PCM or a gateway module) sees abnormal voltage conditions suggesting a short circuit on the Bus G circuit, preventing proper communication between modules connected to this network.
The wiring for Bus G has a short to ground, battery voltage, or an internal short between communication wires, commonly due to chafing, pinch points, or harness damage.
A control module connected to Bus G has internally failed and is pulling the bus voltage low or shorting the communication circuit.
Water intrusion, corrosion, or physical damage at Bus G connectors has created a short circuit or high-resistance connection.
Afterhead unit, remote start, or other aftermarket devices wired into the Bus G circuit may be defective or improperly installed, causing a short.
A shared power or ground circuit supplying modules on Bus G has developed a fault, causing abnormal bus voltages.
The Powertrain Control Module, which often monitors the bus, has failed and is incorrectly reporting the fault - this is the least likely cause and should be diagnosed only after all other possibilities are ruled out.
It depends on the severity. Some vehicles may run but with limited functionality, while others may not start or may stall. Driving is not recommended until the issue is diagnosed, as multiple safety-related systems may be affected.
Replacing the PCM for this code is almost never the correct fix. U0091 indicates a short on the Bus G network itself - the PCM is typically just the module that detects the problem, not the cause. The PCM should be replaced only after all wiring, connectors, and other modules on the bus have been ruled out as the source of the short.
Repair costs vary widely depending on the cause - a wiring repair may cost a few hundred dollars, while a failed module or extensive harness damage could cost significantly more. Diagnosis is essential to determine the actual repair needed before estimating total cost.
The 'G' bus designation varies by manufacturer. Some brands use specific letter designations for different CAN or LIN buses, so the exact network affected differs between manufacturers. The diagnostic approach remains similar regardless of the brand.