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On some of these Navigators there is a factory installed cabin air filter. If this filter becomes plugged it can block the air flow to the vents. The filter is located under the dash on the passenger side in the heater box. You will need to remove the filter and inspect it for debris. If it is real dirty you should replace it. I hope this helps, If I did help you please hit accept as this is the only way we get compensated. If you have any further questions please do not hesitate to ask.
Air all blower speeds inhibited with this reduction ? Does it happen after the AC System has been running ? How does it operate in the heater or vent modes, is air flow still reduced ?