Have any of you WITHOUT a check engine light ever replaced an O2 sensor and seen an improvement in fuel economy? Second, would replacing the front sensor ($57) do anything if the rear one were still original?
I have 115k on the odometer and no check engine light; fuel economy is down a bit to 16 city, 19 highway. While working on the 4Runner I lost the O2 gasket, which is $10 vs $57 for a whole new sensor with the gasket, which is what started this whole thing.