Peugeot 307 exhaust sensor issue

Sounds familiar! I had a similar issue with my old Opel Astra, though it was the O2 sensor near the catalytic converter. The symptoms were almost identical. Have you checked the wiring and the plug connection to the sensor itself? Maybe there's some corrosion or a loose wire.

Yeah, I gave the plug a quick look, didn't see anything obvious. But where should I be looking for corrosion? Is there a specific spot on the exhaust system where these things usually fail?

Well, on my Astra, the corrosion was right at the sensor plug, where it connects to the wiring harness. But exhaust systems are a pain to diagnose without proper tools. Given the symptoms and error code, I'd recommend getting it checked at a workshop. Better safe than sorry, especially with exhaust stuff.

Thanks for the advice! Took it to the shop, and it turned out to be the aged oxygen sensor, just as we suspected. New sensor plus labor set me back 95€. Runs like a charm now!