Alfa Romeo 166 NOx Sensor Causing Check Engine Alert

Had the same symptoms on a 2004 Alfa Romeo 156 last winter. The flashing check engine light paired with emission sensor errors often points to a failing NOx sensor, but the issue can be more complex. Initially thought it was just the NOx sensor, but diagnostic testing revealed corroded wiring connections between the sensor and the engine control unit. The exhaust fumes you mentioned match what I experienced, this happens when the emission control system cant properly regulate the exhaust flow. The root cause in my case wasnt just the NOx sensor itself but the entire pollution control system needed attention. The catalytic converter was partially clogged due to the faulty sensor readings, creating a chain reaction of issues. This turned into a significant repair as both components required replacement to restore proper emission control. For Alfa Romeos of this generation, its a known weak point where the wiring harness connects to the emission sensors. Best to have a complete diagnostic check of the entire pollution control system rather than just replacing the NOx sensor alone. The symptoms you describe are nearly identical to what led to my major repair.

Thanks for sharing your experience with the 156. The emission control system repairs hit me hard on my 166, especially since the air-fuel ratio was way off by the time I got it checked. The catalytic converter damage really complicated things. Quick question, any chance you remember the total repair cost in your case? Also, have you had any other exhaust system problems since getting it fixed? Trying to budget for what might be ahead with mine, since these emission control issues on our Alfas tend to cascade.

Good news, my repair was much simpler than expected. After seeing the mechanic, they found the NOx sensor failure was just due to a faulty cable connection. Fixed it for 95 Euro, which included checking the entire exhaust system to ensure no other pollution control components were affected. Since the repair, the check engine light has stayed off and the emission readings are normal. No other exhaust system problems have cropped up in the months following. Sounds like you unfortunately had more extensive damage due to running longer with the issue, when these sensor failures go unchecked, they can definitely cascade into bigger problems with the catalytic converter and overall pollution control system. The key was catching it early before it could impact other components. Getting that faulty connection fixed right away prevented the kind of air-fuel ratio issues you experienced. These Alfa emission systems are sensitive, but they work well when all sensors are properly connected and functioning.

Based on my experience, it turned out much worse than expected. After getting your reply, I took it to a trusted mechanic who diagnosed multiple issues with the emission control system. The air-fuel ratio was completely off due to running it too long with the faulty NOx sensor. Total repair came to 850 Euro, they had to replace the emission sensor, fix damaged wiring, and deal with a partially damaged catalytic converter. The exhaust system needed some work too since the improper fuel mixture had caused extra wear. Been running fine for about 3 weeks now, no check engine lights and the exhaust fumes are gone. The mechanic said catching these pollution control issues earlier could have saved me at least half the repair cost. Lesson learned about not waiting when emission sensor warnings first appear. At least now I know what to watch for in the future. These Alfa engines are sensitive when it comes to their emission systems, but they run great when everything is properly maintained.