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Heres a schematic for the 04 Tahoe. The symbol on the left of each sensor with the diagonal arrow pointing up and to the right are the actual 02 sensor of the whole sensor. The squiggly symbols to the right of each sensor element are the heaters. So just look at the wire colors and measure across the correct pair at each sensor. Each heating element circuit will have a pink wire on one side of the element. Just touch your meter the corresponding on the sensor, etc. (with wiring harness unplugged from sensor).I cannot tell which, the only pair with a measurement from the Ohm setting.