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.
P0131 error when working engine hard
- Thread starter bladteth
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nd4sped
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Welcome back after a 10 year hiatus, seems that you stop in when you have problem questions. Guess thats ok but stick around more often and might learn a few things 
As for the error, inspect the exhaust and gaskets to ensure there are no leaks. Extra o2 getting into the exhaust stream can cause false positives for the o2 sensor.
Also if the only seems to happen when the engine is under heave load, accelerating during overpassing as mentioned, climbing steep inclines, etc. You may have a leaking/faulty fuel injector that is making the fuel mixture too rich or perhaps too lean. Lean is worse as detonation will cause pitting in the piston face (typically at the edge near the cylinder wall) and cylinder head.
Call local shops and see if they have the ability to flow test your injectors. A lot of times this is a pretty cheap test especially if you remove the injectors yourself.
As for the error, inspect the exhaust and gaskets to ensure there are no leaks. Extra o2 getting into the exhaust stream can cause false positives for the o2 sensor.
Also if the only seems to happen when the engine is under heave load, accelerating during overpassing as mentioned, climbing steep inclines, etc. You may have a leaking/faulty fuel injector that is making the fuel mixture too rich or perhaps too lean. Lean is worse as detonation will cause pitting in the piston face (typically at the edge near the cylinder wall) and cylinder head.
Call local shops and see if they have the ability to flow test your injectors. A lot of times this is a pretty cheap test especially if you remove the injectors yourself.