Smart Roadster O2 Sensor Issues, CEL & MPG Decline

Recently handled similar symptoms on my 2007 Smart ForTwo Coupe with the 1.0L gasoline engine. The check engine light and poor fuel economy perfectly match what I experienced. The diagnostic scan pointed to a faulty exhaust sensor connection. The air-fuel ratio was off due to the oxygen sensor not properly communicating with the engine management system. Initially thought it was a completely failed sensor, but it turned out to be just corroded wiring connections near the catalytic converter. The fix involved cleaning the connection points and replacing a short section of damaged wiring harness. This was significantly less serious than replacing the entire oxygen sensor or catalytic converter. The repair restored proper sensor communication and the exhaust system went back to operating efficiently. Key indicators it was a connection issue: Intermittent check engine light, Fluctuating fuel consumption readings, Engine running rich occasionally, No physical damage to the sensor itself Would suggest checking the wiring connections first before replacing any parts. The symptoms you describe are classic signs of compromised sensor data reaching the computer rather than component failure.

Thanks for sharing your experience with the wiring issue. The symptoms you described match mine exactly, especially the intermittent check engine light and poor fuel efficiency. My mechanic is looking at it tomorrow and mentioned checking the air-fuel ratio first. By the way, do you remember how much you paid for the repair? Also, has everything been running smoothly since the fix, or have you noticed any other issues with the engine management system?

After the repair was completed last month I can confirm everything is running perfectly now. The mechanic found exactly what I suspected, the oxygen sensor wasnt faulty, just the wiring connection needed fixing. The repair cost came to 95€ which included diagnosing and fixing the cable connection near the catalytic converter. The check engine light hasnt come back on and fuel efficiency returned to normal once the air-fuel ratio was properly regulated again. The exhaust system is working as intended with no engine management issues since the repair. Looking back, those intermittent problems were frustrating but the fix was simpler than initially feared. If your symptoms are similar, definitely have them check the wiring connections first before considering full sensor replacement. A bad connection can mimic a failed O2 sensor but costs much less to repair.

So I took my car to the mechanic today based on your advice about checking the wiring first. Good call. They found corroded connections near the catalytic converter, just like you described. The exhaust sensor itself was actually fine. Total repair cost was 120€ for cleaning the connections and replacing a small section of damaged wiring. The automotive sensor readings are now stable and the check engine light has stayed off. Fuel efficiency is already improving after just a short drive home. The mechanic showed me the old wiring, it was pretty bad with visible corrosion near where it connected to the exhaust system. Really glad I asked here first instead of jumping straight to replacing the whole oxygen sensor. Saved some serious money thanks to the forum advice.