I have a 94 GT with 111000 miles on it and one of my O2 sensor's went bad but it sent a code to the check engine light (173)...I swapped out the sensor and it fixed my problem, but if you want to check it, here's the steps out of the Haynes manual:
Allow the engine to reach operating temp and check that the sensor is producing a varying signal voltage between 0.1 and 0.9 volts.
Also, make sure the sensor is getting 12 volts from the battery
You'll have to get the manual to see the diagram to pin check the correct wires, but trust me, IT'S WORTH $15!!!!
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