Check Engine + NOx Sensor Issues: DIY Diagnosis Help

Having dealt with something similar on my 2013 Chevy Sonic, I can share my experience. As someone with moderate DIY experience, I initially thought it was just an exhaust leak, but the check engine light pointed to deeper issues with the NOx sensor affecting the air-fuel ratio. My repair shop found a damaged wiring connection to the NOx sensor, the cable was corroded and making poor contact. Total fix was 95 Euro including labor and parts. The symptoms cleared up right after repair and haven't returned in months. What scanner code are you getting specifically? Could help pinpoint if you're dealing with the same issue. Also, does your exhaust smoke look normal or is it darker/thicker than usual? These details would help determine if it's just the sensor or something else in the exhaust system.

Thanks for the detailed response. The code I'm getting is P0420, and yeah, the exhaust smoke is definitely darker than normal, especially during acceleration. Just had my service done at 98929 KM and this started shortly after. I'm wondering if the catalytic converter might be involved too, since I'm noticing a sulfur smell along with the exhaust leak. The nitrogen oxide readings seem off based on the scanner data. Did your repair shop do any testing of the full exhaust system before zeroing in on the sensor failure? Trying to avoid multiple repair visits if possible.

Just to follow up on my previous experience, when I had similar symptoms with my Sonic, the repair shop did actually run a full diagnostic on the exhaust system, including the catalytic converter. The P0420 code you're getting is different from what I had and could indicate catalyst system efficiency below threshold. The sulfur smell combined with dark exhaust definitely rings some alarm bells. In my case, it was just the NOx sensor wiring, but your symptoms point to potentially more complex pollution control issues. The nitrogen oxide readings being off could be either from a faulty sensor OR from a failing catalytic converter not properly processing the exhaust gases. Since you mentioned this started after service, it's worth checking if anything got disturbed during maintenance. The air-fuel ratio being off can cause both dark smoke and catalyst issues. Based on the mileage you mentioned, you might be looking at both sensor and catalytic converter replacement, that's typically around when these diesel engine components start showing wear. Would strongly recommend getting a full exhaust system diagnostic, wish I'd done that first rather than assuming it was just the sensor. Might cost more upfront but saves money in the long run.

Thanks for the comprehensive breakdown. After hearing about your experience and comparing symptoms, I think I'll take it in for that full diagnostic you suggested. My gut was telling me it might be more than just the NOx sensor, and those details about the P0420 code matching catalytic converter issues make a lot of sense, especially with that sulfur smell I'm getting. Your point about the check engine light and exhaust issues needing a complete system check really hits home. Going to schedule a diagnostic next week and have them look at both the emission sensor readings and catalytic converter condition. Rather deal with everything at once than have to go back multiple times. Really helpful to know about the air-fuel ratio connection to the dark exhaust smoke too, gives me a better picture of what I'm dealing with. Just hoping it's not going to need a full catalytic converter replacement, but at least I'll know what I'm up against after the diagnostic.