Q: Why does my 2007 GMC Sierra Classic shifts from 1st gear to overdrive?

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Truck will shift from 1st to overdrive, what's the cause?

My car has 158890 miles.
My car has an automatic transmission.

Hello. Most of the time this is caused by a pressure issue within the transmission. It can also occur of there is a major internal failure as that is what the computer does to try and protect the transmission. I would scan the computer for any codes first. Any codes would tell me what the computer may see as a problem. If there are not any codes then I would check the pressures out in the transmission. If you want to have this properly diagnosed then you should take the vehicle to a transmission specialist.

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