What it means
A control module has received data from the gear shift control module that it considers invalid or implausible. Gear selection or related functions may be affected; the specific effects depend on the vehicle.
At a glance
- Usually lights the warning light of the system that lost communication
- The engine or gearbox may go into limp mode
How it works
The gear shift control module reports the driver's gear selection to other modules, often on vehicles with electronic shift controls. U0404 is set when a receiving module judges that selection data implausible or incompatible with the expected configuration.
Common causes
Most likely first.
- Implausible or incorrect gear shift control module data
- Software or configuration incompatibility
- Intermittent network data corruption
Symptoms
- Gear selection or related functions may be limited or unavailable
- Related warning indicators may illuminate
- A diagnostic scan may show related communication codes
How to diagnose it
- Scan all modules and save freeze-frame data, noting which module logged U0404 and any gear selector or transmission codes.
- Compare live gear selector data with the position shown at the shifter and the gear the vehicle actually engages.
- Inspect the gear shift control module, selector, and network connectors and wiring for damage, corrosion, or loose terminals.
- Check module power, grounds, and network communication for interruptions that could corrupt the selected-gear message.
- If electrical checks pass, verify the shift control module's software and configuration and diagnose any selector or module faults indicated by related codes.
Common fixes
- Repair a loose or damaged shift control module connector or network wire.
- Correct the shift control module configuration or software if incompatible.
- Repair a faulty gear selector or shift control module when testing confirms it is sending incorrect data.
- Replace an incorrectly specified module and program it for the vehicle.
Can I keep driving?
Gear selection may be limited or disagree with the displayed position, creating a risk that the vehicle will not move or hold as expected. Do not drive if the selected gear is uncertain or the vehicle shifts unexpectedly; have it checked before continued use.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.