Cadillac XTS NOx Sensor Issues and Exhaust Leaks Alert

Having some experience with similar issues, I can share what happened with my 2012 Cadillac CTS. The symptoms you describe match exactly what I encountered last year. The check engine light came on, and initial diagnostics pointed to the NOx sensor failure. After inspection, the workshop found that the issue was actually simpler than expected, a damaged cable connection to the sensor rather than a complete sensor failure. The fix involved repairing the wiring harness and ensuring proper connection to the pollution control system. Total repair cost was 95€, which included diagnostic work and the cable repair. Much better than replacing the entire NOx sensor or catalytic converter components. Before suggesting specific repairs, how many miles/kilometers are on your XTS? Are you noticing any performance issues or unusual exhaust sounds? What specific error codes is your system showing? Have you had any recent exhaust system work done? Getting this info would help determine if your issue matches what I experienced or if you might be dealing with something different in your exhaust system.

Last service was just 2 months ago at 164000 KM. Not noticing any actual performance issues, but there is a slight rattling sound from the exhaust area. Diagnostic shows error code P0420, which typically relates to the catalytic converter system efficiency. The exhaust system has been completely stock since purchase, with no previous repairs needed until now. The pollution control components seem to be the main concern, as the check engine light persists even after clearing the codes. Would be great to hear if anyone has experienced similar symptoms with relatively high mileage XTS models.

Thanks for providing those details, sounds very similar to what I experienced on my XTS after crossing 160000 KM. The P0420 code combined with the rattling suggests this might be more complex than just my previous wiring issue. In my case, after the initial wire repair, the problem returned within weeks. Further diagnosis revealed the air-fuel ratio was off, affected by both a failing NOx sensor and a deteriorating catalytic converter. The emission sensor readings were inconsistent, which explained the persistent check engine light. The complete fix involved: Replacing the NOx sensor (320€), Catalytic converter repair (580€), New gaskets for the exhaust system connection points (90€) The stock exhaust system typically lasts around 150000-170000 KM on these models, so the timing fits. The rattling you mention is often a sign the internal catalyst material is breaking down. Before committing to major repairs, have a workshop check the exhaust system connections and the air-fuel ratio. Sometimes adjusting the mixture can extend catalyst life and resolve sensor readings. The complete repair restored normal operation, and the check engine light hasnt returned in 15000 KM.

Thanks everyone for the detailed responses. After reviewing the suggestions and comparing my situation with similar experiences, I decided to get a complete diagnostic done at a local workshop. The check engine light and error code P0420, combined with the high mileage and rattling sounds, definitely point toward catalytic converter issues. Will schedule the inspection next week, focusing on both the emission sensor readings and the condition of the pollution control components. Good to know the typical lifespan of the exhaust system aligns with my current mileage, helps explain the timing of these issues. I appreciate learning about the possible connection between the NOx sensor and catalytic converter efficiency. The suggested repair costs give me a solid baseline for what to expect. Will update once I get the official diagnosis and repair estimate from the workshop.