Peugeot 406 transmission control unit issue

The symptoms you describe certainly point towards a potential issue with the transmission control unit, especially given the stored error code. However, it's important to rule out other possibilities first. Have you checked the transmission fluid level and condition? Low or contaminated fluid can cause similar symptoms. Also, have the wiring and connections to the transmission control unit been inspected for damage or corrosion? These are simpler issues to address before considering a faulty control unit.

Thanks for the reply. I haven't checked the fluid level myself, but the mechanic I usually go to said it looked okay a while back. As for the wiring, I didn't check it myself. Where exactly should I be looking for those connections? Is it easy to spot, or is it buried deep?

While the mechanic's assessment of the fluid level is helpful, a recent check would provide more definitive information. Regarding the wiring, the transmission control unit is typically located in the engine compartment, often near the transmission itself. The connections to inspect are the wiring harness connectors that plug directly into the control unit. Look for any signs of corrosion (a white or greenish deposit), damage to the wires themselves, or loose connections. Given the complexity and potential for misdiagnosis, I suggest seeking a professional diagnosis from a qualified workshop equipped with the appropriate diagnostic tools and expertise. Defective automotive ECUs aren't easy to self-diagnose and the symptoms could indicate more problems.

Thanks a lot for your help! I took it to a workshop, and you were right – the transmission control unit was indeed defective. The repair cost me around 1000€, but at least it's finally fixed.