Silverado CEL: O2 Sensor vs Exhaust Leak Diagnosis

Had the same problems with my 2019 Sierra, really similar setup to your Silverado. Been working on cars as a hobby for a few years, so I'm fairly familiar with these issues. The symptoms matched my case exactly, CEL, rough idle, and cabin fumes. Turned out the lambda sensor (oxygen sensor) was completely shot. The bad readings were causing rich fuel mixture, explaining the strong exhaust smell. Had to get it replaced at the shop since it needed special diagnostic tools. Total bill came to 375 for parts and labor. Since then, the truck's been running smooth with no exhaust issues or check engine lights. What's your current mileage? And have you noticed if the fumes get worse at idle or when accelerating? This could help pinpoint if you're dealing with just a sensor issue or if there might be problems with the catalytic converter or exhaust pipe connections too.

Thanks for the input. I'm at 41443 KM and just had the regular service done last month. The fumes are definitely worse during idle, and the exhaust leak sound is more noticeable when cold starting. Just checked under the vehicle and noticed some black residue around the catalytic converter connection points. Guessing there might be a failing gasket or loose connection causing the exhaust leak. The check engine light started flashing yesterday, so probably need to get it looked at ASAP before it causes more damage.

Definitely get it checked soon, those exact symptoms on my Sierra led to bigger issues when I waited too long. The black residue around the cat is a clear sign. Mine had similar buildup, and it turned out the exhaust leak was actually damaging the oxygen sensor, which then threw off all the engine readings. When my check engine light started flashing (just like yours), it was the exhaust leak causing incorrect readings from both the O2 and NOx sensors. The ECU was getting confused by the bad data and running the engine way too rich. That's why you're getting those strong fumes, especially at idle. Had my mechanic do a full exhaust system inspection. They found three issues: Failed exhaust gasket at the cat connection, Damaged oxygen sensor from the leak, Corroded bolt on the exhaust flange Total fix was around 550 for everything including labor, but catching it early prevented catalyst damage. With those flashing CEL warnings, you'll want to address it quickly to avoid the same expensive catalyst replacement I nearly faced. Make sure they check all the sensor connections too, the exhaust leak can trick the system into thinking there's a DEF system problem even though it's just bad readings from the leak.

Thanks everyone for the advice. Just wanted to update that I took it in this morning given how serious the symptoms were getting. The diagnosis confirmed both an exhaust leak and oxygen sensor malfunction, exactly what was suggested here. The mechanic found a cracked exhaust pipe connection right before the catalytic converter plus a failing O2 sensor. The EGR valve also needed cleaning since all the exhaust issues were causing carbon buildup. Total repair came to 625 including parts and labor, which seems fair considering the multiple issues. Truck's running perfect now, no more check engine light, rough idle is gone, and cabin is completely free of exhaust fumes. Really glad I didn't wait any longer as the mechanic mentioned the leaks were starting to affect other sensors and could have damaged the catalytic converter if left unfixed. Thanks again for helping me diagnose this before it turned into something much more expensive. Will definitely keep a closer eye on any exhaust-related symptoms going forward.