The cooling fan"expert" is here
Probably, the low speed circuit has failed. Remove the 2 relays, key off. If you jump 3-5 on the green relay's socket,the fan should run low speed. Then jump 3-5 on the black high speed relay's socket the fan should run high. If the 2 speeds work, with strong airflow then the fan circuit/resistor is OK. (You could pull the ECT sensor plug, see if the fan comes on and stays on. That will set the CEL light.) The Pin 7 of low speed relay goes to PCM (Powertrain Control Module) which triggers the relay, and could be a wiring problem there. The a/c should trigger the fan after a short delay. Fan first, compressor next is how mine (V6) works. If the fan does not work, the a/c will overpressure shortly, turn a/c off asap. Here is a V6 schematic, the Zetec is the same, 1 fan motor instead.
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