Smart Fortwo Gearbox ECU Failure Symptoms and Fixes

Had an identical transmission control issue with my 2004 Smart Roadster last summer. Have some experience working on cars but this was definitely beyond DIY territory. The symptoms matched yours exactly, erratic shifting, no paddle response, and the car computer showing transmission fault codes. After limping to the workshop, their transmission diagnosis confirmed the control module was the culprit, not the mechanical components. The repair involved: Full electronic diagnosis of the transmission control unit, Replacement of the defective control module, Recalibration and coding of the new unit, Testing all transmission functions Total cost was 1000€ including parts and labor. The gearbox issues cleared up completely after the repair. Before suggesting specific fixes, could you share: Current mileage? Any previous transmission work done? Exact error codes from the car computer? When did symptoms first appear? Does the problem get worse when the engine is hot? This would help determine if your situation is similar and rule out other potential causes. These automated manual transmissions are quite sensitive to their control units, but its worth confirming the diagnosis thoroughly.

Thank you for the detailed response. Just had the 120000km service done last month. The transmission diagnosis seems spot on, my symptoms match exactly what you described. The error codes started appearing about 2 weeks ago, always after 15-20 minutes of driving when the automotive ECU is fully warmed up. The transmission control issues definitely worsen with heat. No previous work done on the automatic gearbox, running all original parts. Starting to think the control unit replacement might be the way to go, especially given your positive experience. 1000€ sounds reasonable compared to a full transmission rebuild. Will get it properly checked at a workshop that specializes in Smart transmission control modules. Rather catch it early before any mechanical damage occurs to the gearbox itself.

Thanks for the additional details. The symptoms getting worse with heat really confirms my suspicion about the transmission control unit. After my repair, I learned these control modules often fail due to heat stress after around 100000-120000km, exactly where yours is at. Its good you caught the electronic issues early. On my Smart, waiting too long with a faulty control unit caused additional wear on the automatic gearbox components. The transmission problems started similarly with occasional problems, but quickly developed into constant issues. One tip: when you get the replacement unit, make sure they flash the latest firmware. My workshop initially skipped this step and the transmission control wasnt working optimally. After the proper update, the shifting became noticeably smoother. The car computer should show clean readings after the repair. Worth checking the transmission diagnostic codes again after a few days just to confirm everything is properly resolved. Mine has been running flawlessly for 40000km since the control module replacement. Keep us posted on how the repair goes. Always useful to track these transmission control unit cases, especially with these particular gearbox designs.

Thanks for sharing your experience with the control module repair. After getting several quotes, I decided to go with a specialist workshop that has extensive experience with Smart transmission diagnosis. Booked in for next week. The car computer is now showing additional fault codes, and the transmission issues are getting more frequent, especially in stop-start traffic. Really glad I didnt wait longer to address the electronic issues. Will document everything including the automotive ECU readings before and after repair. Should help others facing similar automatic gearbox problems. The 1000€ estimate seems to be the going rate, got similar quotes from three different places. Appreciate the tip about checking the firmware update. Will make sure to specifically request this when they install the new transmission control unit. Planning to do some thorough testing after the repair to ensure all shifting functions are working correctly. Will update the forum once the work is complete. These control module failures seem quite common on these transmissions, so documenting the repair process should be helpful for other owners.