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U0291 — Lost Communication Gear Shift Control Module B

Generic OBD-II · all makesNetwork / communication

U0291 is a network communication code set when the powertrain control module (PCM) or another primary gateway module stops receiving messages from the Gear Shift Control Module B over the vehicle's CAN bus. The 'B' designation indicates the vehicle has more than one gear shift control module, typically found in vehicles with electronic shift-by-wire or electronically controlled shift mechanisms. The code points to a communication failure, not necessarily a mechanical transmission fault.

Symptoms

What causes it — most likely first

1. Faulty gear shift control module

The gear shift control module itself is the most common failure point. Internal electronics can fail due to heat, vibration, or moisture intrusion, causing it to drop off the network.

2. Wiring or connector problems at the module

Corroded, loose, or damaged connectors and wiring harnesses to the shift module are very common. Inspect the connector pins for spreading, corrosion, or pushed-out terminals.

3. Loss of power or ground to the module

The module requires clean battery voltage and a good ground to operate. A blown fuse, broken ground strap, or corroded ground point will prevent the module from communicating.

4. CAN bus network issues

A short, open, or high resistance on the CAN high or CAN low circuits anywhere on the bus can knock the shift module offline. This often produces multiple U-codes.

5. Software or calibration issue

Outdated or corrupted module software can prevent proper handshake with the rest of the network. A reflash or update may resolve intermittent communication issues.

6. PCM or gateway module failure

Rare for this specific code, but a failing PCM or network gateway can fail to recognize the shift module. Only consider this after all wiring, power, and the module itself have been thoroughly ruled out.

How it's diagnosed

  1. Scan the vehicle with a full-system scanner to confirm U0291 and check for related U-codes, which can indicate whether the issue is module-specific or network-wide.
  2. Inspect the gear shift control module connector for corrosion, bent pins, loose terminals, or damage; repair as needed before proceeding.
  3. Check the power feed and ground to the gear shift control module using a multimeter, including verifying the related fuse is intact and the ground point is clean and tight.
  4. Test CAN bus integrity by measuring resistance between CAN high and CAN low at the module connector (typically around 60 ohms with the key off) and check for short to power or ground.
  5. Attempt to communicate with the shift module directly using the scan tool; if it does not respond but wiring and power are good, the module itself is likely faulty.
  6. After repairs, clear codes, cycle the ignition, and perform a road test to confirm the code does not return.

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
Related codesU0289 — Lost Communication Dcac Converter ContU0290 — Lost Communication Active Aerodynamic U0292 — Lost Communication Drive Motor ControlU0293 — Lost Communication Hybridev PowertrainU0294 — Lost Communication Powertrain Control U0295 — Lost Communication Acac Converter Cont

Frequently asked

Can I keep driving with code U0291?

It depends on the vehicle. Some will operate normally with reduced shift quality, while others may enter limp mode or refuse to start due to shift interlock logic. Address it as soon as practical.

Is the transmission itself bad when this code appears?

Not usually. U0291 indicates a communication problem with the shift control module, not internal transmission wear. The transmission can often be fine once communication is restored.

Do I need to replace the PCM to fix U0291?

No. Despite what some sources suggest, the PCM is almost never the cause of U0291. The gear shift control module itself, its wiring, connectors, or power supply are the realistic culprits. Replacing the PCM for this code is costly and rarely resolves it.

Does U0291 clear on its own?

It may clear if the underlying issue is intermittent, such as a loose connector or temporary voltage drop, but the code will typically return if the root cause is not repaired.