“Gearbox Fault” Error and AL4 Transmission Solenoid Valves (Citroën C4)

This is classic transmission behavior. AL4 entering Emergency Mode (Limp Mode). When the transmission computer detects a fault (usually a difference between the desired and actual oil pressure), it locks the car in 3rd gear to protect the internal gears.

• Focused Model: Citroën C4 (Pallas, Hatchback equipped with 4-speed automatic transmission).
• Location of Protection: The fuse that powers the Automatic Transmission Electronic Control Unit is located in BSM (Engine Service Box), under the hood.
• Correct Fuse: It’s the fuse. F10, with amperage of 15A.

Causes of the Problem:

1. Fuse F10 (15A): If this fuse blows, the transmission control unit loses communication with the car, and it will immediately enter fail-safe mode.
2. Solenoid valves: In the vast majority of cases of “Gearbox Fault,” the problem is wear and tear on the two pressure modulating solenoid valves (BorgWarner). They become clogged or lose their seal.
3. Degraded Oil: AL4 transmission fluid is not “lifetime” fluid. Failure to change it leads to dirt buildup that clogs the solenoid valves.

Technical Tip: If the car returns to normal after you turn the key off and on, the fuse… F10 It’s fine, and the defect is intermittent in solenoid valves or low oil level. If the error is permanent and the scanner does not “read” the transmission, check the fuse. F10 at BSM.