Rep Power: 0

I drive a Honda Civic EM2 with JDM D17A VTEC engine.

I failed the emissions test recently because the P0171 code appeared during the test. CO level was at 0,73% (max is 0,2%) and lambda value was 1,023. I still haven't been able to find the cause of the code.

A friend of mine (mechanic) said I have no vacuum leaks and that it's not burning oil.

O2 sensor flactuates between 0,1 and 0,9v in idle and when accelerating. STFT is really high during idle (15-20%). It goes to 0% while cruising and generally spikes to 15-20% while accelerating. Consistent reading regardless of RPM.

IAT shows the correct temperature I think, around 30-40°C during idle and 19-27°C when driving which aligns with outside temperature.

I had a P0113 code but probably because my friend unplugged the sensor once. After clearing neither the P0171 nor P0113 have come back but STFT stay high and emissions probably too.

Any ideas what it could be? What should I check next?

 
Rep Power: 223

You probably need to change the primary o2 sensor. Only use denso. If you purchase online be careful, sometimes theres counterfeit parts.