Q: Gears engage after 05 Expedition sits for a day but gears stop engaging after short drive.

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Automatic Transmission. My check engine light came on, code P0506. I had it read at auto zone. Days later, while driving, the gears slowly stopped engaging. At first, it was taking a long time for it to increase speed and within 2 minutes it stopped moving at all. When you press gass pedal, truck only revs. Both reverse and drive. I cleaned throttle body the next day as auto zone rep recommended. It worked at first. Gears engaged. I took a short (1/4 mile) drive around neighborhood and when I came to a stop sign, the gears stopped engaging again. It sat for 2 days. I tried to drive it again. The same thing happened: it worked fine for short drive and then gears stopped engaging. What could this be? Why would it work after sitting for a while and then go out again?

My car has 200000 miles.
My car has an automatic transmission.

Hi There, Code P0506 relates to the idle air control valve, specifically that the idle speed is less than the target idle speed by 100 RPM or more. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle’s computer and will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. This is also an important function when starting the motor as it allows the motor to run and idle on it’s own once the motor fires. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed with the help of the idle air control valve making the transition from a higher RPM back down to idle speed while adjusting the air/fuel ratio constantly to allow this to happen smoothly. When the engine RPM drops below the normal range of about 750-800 RPM, this often times will cause the engine to stall indicating a dirty or faulty idle air control valve. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.

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