I have 7th gen Civic (EU) with 1.6 l engine. I've replaced upstream O2 sensor after heating element failed in the old one (error P0135 if I remember correctly). It worked fine for a while, but I rarely used the car and mainly took it easy. After a few longer trips check engine light started to appear, always with error P0131. I tried to replicate the problem and it only happens when the engine is worked hard for longer periods, eg. when overtaking or going uphill. If I don't reset it using ODB reader, it usually goes off the next day or so. I checked the wiring and it's intact. Idling is smooth and steady. The car did not seem to lose power. Fuel economy is pretty much the same. No strange smells from exhaust. There was a leak in rear muffler, which I replaced, but it did not change the behaviour. I've also topped up engine oil as it was slightly below correct level on the dipstick - again with no change. What should I check next? Thanks.