F 150 4WD Pickup V8-5.4L CNG VIN M (2001)
P1640 - P1644: Testing and Inspection
P1641
For diagnosis of code P1641 refer to chart
P1518 - P1781
If there is no link in the chart then:
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This code is not a valid code for this vehicle
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Or information published by the manufacturer for this vehicle did not include this code