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I haven't used the car for over 3 months and I started it a couple days ago with no problem but today I couldn't manage to start the engine so I had to push start it.

Since then, both the check engine light and the epc light are on. In the beginning the throttle wasn't really working at all, the rpm wouldn't change no matter how hard I pushed it, but restarting the car fixed that issue.

The EPC and check engine lights are still on though and my obdII reader gives me the p0638 code. What could be the cause? Could it just be because I push started the car? Should I just clear the code?

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This is not a definitive answer, but a few thoughts... The code you give is from "Throttle Actuator Control Range/Performance" - in other words, an electronic sensor gave a weird reading. If this was my car, I would clear the codes, go on with my life and see if the code came back.

Since you push started the car, it didn't have enough electric power to start, and all the sensors might not have been acting right. Also, your car's computer isn't expecting the pattern of input you get when you push start rather than normally start. That doesn't mean it's bad - just unexpected, and it could look like an error.

Also, and again--no references to back this up, but some codes are "sticky" and some go away on their own. Certain engine parameters might set a code, and when they look better on the next engine run, they get cleared. Other codes, once set, will not go away on their own even if the odd reading has gone away. A trouble code dealing with something as important as throttle position or actuation sounds like a code that might be sticky. If the throttle is giving odd readings, someone should check it out to avoid your VW going all Audi 5000 on you.

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    A further comment - if it needed a push start the battery may be shot and low voltage to various control modules may be causing really odd symptoms. Make certain the battery is fully recharged (or replaced) before spending too much time with extended diagnosis. – SteveRacer Jan 30 '18 at 6:06

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