need help with 02 sensor/ afr gauge hook up

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was having problem with afr gauge after dump pipe installed. found out wire was broken on o2 harness . question. o2 sensor has 3 wires 2 white one blk in middle . installer had afr gauge hooked to one of the white wires is this correct? the sr harness has three blk wires . how would i tell which wire is which? I thought the white wires where heater wires and ground. and the middle wire would be the sensor wire to hook up to. would it hurt the afr gauge if i hook it up to the wrong wire to test it?