Black Smoke & Power Loss: Infiniti JX Mystery Solved?

I had almost identical issues with my 2015 Infiniti QX70 last summer. The symptoms matched yours exactly rough idle, black exhaust, poor fuel economy, and that annoying check engine light. Turned out to be a faulty lambda sensor heater, just as you suspected. Got it fixed at the dealership because these sensors are pretty critical for proper engine management. The repair involved replacing both upstream O2 sensors since they tend to fail around the same time. It wasn't a super complex job, but it's important to get it done right because bad sensors can damage your catalytic converter if left too long. After the fix, the engine ran smooth as new, fuel economy went back to normal, and no more black smoke. It's definitely worth getting it checked out soon I learned that ignoring these symptoms can lead to bigger problems down the line. If you're near any Infiniti specialist shops, I'd recommend going there over a general mechanic. They'll have the right diagnostic tools and experience with these specific issues.

Thanks so much for sharing your experience that's really helpful! Sounds exactly like what I'm dealing with. Hey, do you remember roughly how much you ended up paying for the repair? Also, has everything been running smoothly since then, or have you noticed any other issues pop up? I'm trying to budget for this and want to make sure I'm prepared if there might be related problems down the road.

Hey there! Yeah, I went ahead and got it fixed at the dealer I mentioned ended up costing me 375€ for the lambda sensor heater replacement. Not the cheapest fix, but totally worth it! Since getting it sorted, my QX70's been running like a dream no more black smoke, engine's super smooth, and my fuel economy is back to where it should be. Honestly, it's like driving a different car! Haven't had a single issue pop up since the repair, which was about 7 months ago now. The mechanic confirmed it was definitely the faulty lambda sensor heater causing all the drama. Once they replaced it, all those annoying symptoms the rough running, power loss, everything completely disappeared. If you're seeing the same symptoms, I'd say go for it. Better to fix it now than risk more expensive problems later on. Just a heads up if you decide to get it done, maybe call around to a few Infiniti specialists first. I got lucky with my dealer, but prices can vary quite a bit between shops.

Thanks for all the details! I actually just got back from the mechanic yesterday. Took your advice and found a certified Infiniti specialist nearby. You were spot on it was the lambda sensor heater causing all the trouble. Ended up paying around 400€, which wasn't too bad considering the headache it was causing. They got it done in about three hours, and wow, what a difference! The engine's running super smooth now, no more black smoke, and my fuel economy has already improved. Funny thing is, the mechanic told me the exact same thing about fixing it sooner rather than later to avoid damaging the catalytic converter. Really glad I didn't put it off any longer. The guy seemed really knowledgeable and even showed me the old sensor it was pretty worn out. Thanks again for pushing me in the right direction with this. Definitely feeling relieved now that it's all sorted. Time to enjoy my smooth-running Infiniti again!