2004 Toyota Sienna Chech engine light

Engine Mechanical problem
2004 Toyota Sienna 4 cyl Front Wheel Drive Automatic 38k miles

I have a 2004 Toyota Sienna LE with about 38k miles on it. This afternoon, the check engine light came on. Of course the dealership wants $90 to diagnose the problem and them being a dealership, I'm sure they will find something wrong with the car which will cost a pretty penny to fix and probably won't fix the problem I came in for.

I checked the gas cap which was on tightly and I popped the hood. I don't know how or why but the engine cap was off and all the way on the left side of the car. Unbelievable. The last time I had an oil change was back in February and we haven't been under the hood since so I don't know if the cap has been off this whole time.

Can it really be as simple as a cap that isn't on?

Thanks in advance!