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International Automotive Technicians Network
Re: 2004 PETE 379 C-15 ENGINE FAN ON
Posted to HD/Fleet Forum on 11/27/2020

Sounds like a bad engine computer temp sensor. Many systems are designed to run the fan full time by default if the sensor has shorted or become open circuited. The PCM is getting a bad input.

Also check AC high side transducer or switch if it has one. These... Login to read more.