Sonic O2 Sensor Issue

I had a similar problem with my Sonic. The O2 sensor can definitely cause those symptoms. Was the error code specifically pointing to a faulty sensor, or was it a more generic code? Which sensor is the error related to? Knowing that will help narrow down the problem.

The error code specifically mentions the upstream O2 sensor. I've cleared the code a couple of times, but it returns after a few drive cycles. Given that, and the increased fuel consumption, does it still sound like the sensor itself, or could it still be a wiring issue that's intermittent?

Given that it's a specific code and returns, it's highly probable the sensor is failing. However, intermittent wiring issues are a pain to diagnose remotely. I strongly suggest taking it to a qualified workshop, as they have the right tools to verify the sensor signal and wiring harness integrity. That way you avoid guesswork and potentially replacing a perfectly good sensor.

I took it to my mechanic, and you were right. It was the upstream O2 sensor. They replaced it, and the check engine light is gone, and my fuel consumption is back to normal. Total cost was 95€, including the sensor and labor. Appreciate the advice!