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Hi , Had the air on today and I can't get hi speed. I get the first three but not hi. What do you guy's think it is ? I have front and rear air, the rear works fine. Truck is a 2002 SLT + 5.9 4x4. Thanks...
Check the resistor located under the dash / glove box area. Typically when they fail the blower will only work on high speed, but that's the number one place to start troubleshooting. If you have a hard time removing the connector from it look closely at the contacts and see if they're melted / corroded. If so, splice the harness with a replacement connector from the dealer.
I just replaced my resistor in the winter. I only had the two highest speeds since I had bought the truck in '06. It completely failed in the winter. Once I replaced it, I have all my speeds back. It was $15 from AutoZone.
I would spend the $15 on the resistor before jumping on the harness.