2002 Toyota Corolla • 177 miles

I have a 2002 Toyota corolla which idles very rough in reverse for about the first 5 mins when I first start the car, after that it runs/drives great. I have posted previously and I was told to possibly check for an engine vacuum leak, what does the main vacuum switch do and is this in combination with an EGR valve? Any other ideas? The CEL first came on after I cleaned the throttle body with TB cleaner.

I do have a CEL on, but like I said the drivability is very good.
The code is: PO300
August 23, 2013.

P0300 is a misfire these are very hard to diagnose even for a pro and he should look at it. It can be a bunch of things causing that.

Aug 23, 2013.