U0291 code meaning
Lost Communication With Gear Shift Control Module B
What U0291 means
With shift-by-wire, the driver's lever, buttons or dial is an electronic module that sends the selected range to the transmission over the network. Gear shift control module B is a second unit of this kind; U0291 means the transmission controller has stopped receiving its messages. Allison documents the pair for its sixth-generation controls of the 1000 to 4000 series, released in 2021: U0103 covers gear shift module 1, U0291 module 2, with separate codes for incompatibility (U0333) and invalid data (U0592) of the second selector. Mercedes-Benz uses U029100 on the 8-speed dual-clutch transmission 724.1 for a communication fault with the electronic selector lever module. Range changes requested at that selector are then ignored; the transmission typically stays in or returns to a safe range.
U0291 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0291
- Gear selection at the selector has no effect
- Transmission warning or flashing range display
- Vehicle cannot be shifted out of Park or Neutral
Common causes of U0291
- Most likelyLoose or corroded connector at the second selector
- PossibleBus line to selector B chafed or interrupted
- PossibleSelector variant or software not matching the transmission calibration
- Less likelySelector module B internally failed
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Gear shift control module bVehicle bus wiring
What each part of U0291 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 91 | The exact fault in that area. |
What to do next
- Note U0291 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0291
What does U0291 mean?
U0291 means "Lost Communication With Gear Shift Control Module B". With shift-by-wire, the driver's lever, buttons or dial is an electronic module that sends the selected range to the transmission over the network. It is a generic code, so it means the same on every make.
Is it safe to drive with U0291?
Drive gently and only to a workshop. With U0291 the car can go into limp mode (reduced power) to protect itself.
Will U0291 fail an emissions test?
U0291 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0291 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U0291 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.