Smart Roadster Fails OBD Test Despite Running Fine

I faced similar readiness monitor issues with my Smart Forfour 2006. The onboard diagnostics system needs to complete specific drive cycles to set all monitors ready. When running car diagnostics last month, it showed unset readiness for the catalyst and O2 sensor monitors. The speedometer errors often link to a failing speed sensor, but in my case, it turned out more complex. The catalytic converter was deteriorating, preventing the OBD system from completing its self-tests. Here are key steps before visiting a mechanic: Clear the error codes, Drive for at least 20 minutes at steady highway speeds, Include some city driving with multiple stops, Let the car idle for 5 minutes after a warm engine, Ensure fuel tank is between 1/4 and 3/4 full If system readiness remains incomplete after following these steps, you likely have a genuine emissions system issue. The speedometer errors combined with incomplete readiness monitors strongly suggest catalyst or oxygen sensor problems, which did require professional repair in my situation. This was a serious repair case, as failing catalytic converters can damage other engine components if left unchecked. Best to have a qualified mechanic perform thorough car diagnostics rather than continuing to drive with the issue.

Thanks so much for sharing your Smart Forfour experience. Your catalytic converter issue sounds very similar to what I am dealing with. Did you remember roughly how much the repair ended up costing? My check engine light started flickering occasionally since posting, which makes me think the catalyst might indeed be the culprit. Have you had any other emissions-related problems since getting it fixed?

After getting my Smart Forfour properly diagnosed, the good news was the readiness monitors issue cleared up without needing major repairs. The OBD diagnostics revealed no actual catalyst damage, just a computer reset was needed to complete the system readiness tests properly. The mechanic discovered the previous error codes were triggering false readings in the exhaust system monitoring. A full vehicle inspection and sensor check confirmed everything was actually working fine. The flickering check engine light you mentioned sounds very similar to my situation. The best part was the repair ended up costing 0€ since it just required clearing and resetting the system. No parts needed replacement. The car has passed all emissions tests since then with no problems. For peace of mind, I would still recommend having a professional run complete car diagnostics to verify your catalyst is truly okay, especially given the age and mileage. But there is a chance you might get equally lucky with just needing a computer reset to resolve the readiness status.

After reading about your zero-cost fix, I decided to visit a different mechanic for a second opinion. The visit was eye-opening, turns out my vehicle inspection revealed the check engine light and system readiness issues were caused by a loose gas cap and a dirty mass airflow sensor. The mechanic cleaned the sensor and properly tightened the gas cap. Total cost was 85€ for diagnostics and cleaning. After driving the recommended cycle, all readiness monitors completed successfully. The car passed the emissions test yesterday with no problems. Such a relief it was not the catalytic converter. The exhaust system checked out fine during the inspection. The speedometer errors also disappeared after the sensor cleaning. Guess sometimes the simplest solutions really are the answer. This experience taught me to always get a second opinion when car diagnostics seem unclear. Will definitely keep this mechanic for future repairs since they took time to properly diagnose rather than jumping to expensive fixes.