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Hi All, Got the emissions test failed with high CO emissions at 2500RPM. Took the car to Mr Dhammika at Cotta road and analyzed the emissions. High CO situation happens after letting off gas from high RPM (2500 - 750). Not at sitting idle or at high rpms, only when coming to idle from high RPMs. His suggestion was to replace the MAF, which I was not so sure of. Anyways I failed again with another used MAF (returned and got my cash back). Then I bit the bullet and purchased a scanning software for my car and analyzed the readings myself. The O2 sensor reads high voltage when coming from 2500RPM to 750RPM. The thing is, even though the RPM drops to 2500RPM in a second, the O2 sensor voltage remains high (rich) for like 20 seconds. The short term fuel trim values shows the ECU is trying to lean the mixture by reducing fuel. This almost goes to -20 range and then only the O2 readings goes to lean and start switching between lean and rich. Please have a look at the below graphs and help me to identify the issue.. Engine Nissan QG16DE EFI.