Does anyone have any experience spotting the characteristics of a lazy oxygen sensor? I have a feeling the oxygen sensor in my beater is not acting as it should and is throwing off my long term fuel trim. I'll see if I can post a couple examples of what it looks like while warm idling, under heavy load at 60mph going up a hill, and cruising at 60mph with an average load on a flat surface. You can see in the first image while idling that the waveform appears to peak at 0.8 volts, with a low of 0.1 volt. It seems to be more erratic than it should be just for idling... In this shot this how the waveform typically looks like under heavy load at about 1700 RPM, you can see that it really doesn't peak higher than 0.7 volts and really doesn't go lower than 0.2 volts. If I understand a proper function O2 sensor I think the waveform should be constantly going between 0.9 and 0.1 volts Here is a shot of cruising at 60mph, about 1700 RPM on flat roads with little load. Pretty similar to the above The car is a 98 Olds Intrigue, series 2 3800 I just picked up, has 148,000 miles, dont know much history other than the sensor is likely original and I just replaced the LIM gaskets and replaced the upper plenum. My long term fuel trims around about +12-15 at idle and +7-9 cruising. This says its probably a vacuum leak, but I have checked, and I have great idle vacuum too. Anyone have any opinions?