A code 123 means that the TPS voltage is out of range. Hook up a digital voltmeter to the green (+) and black (-) wire of your TPS (The black thing on the top of your throttle body). Turn the key into the on position and the voltage should be .9 to less than 1 V. Move the throttle slowly toward WOT and see if it goes up smoothly to about 4.6 V. If it doesn't, replace it.
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351W 89 Mustang GT Convertible