Plse Help! Check Engine Light and Heater Problem??

I have a 'very' high milage 2004 Prius... earything worked 'perfectly' until couple of week ago.. the check engine light came on.. I read this could be a leak in emission system somewhere, specially the gas tank cap. After I made sure the cap is tight, I fueled up and drove for 25min..turned car off..(I did this 4 times) .. well, the light went off. A few days later the check engine light came on again, I was able to make it go off again by doing the above.. When the 3rd time the light came on, it stayed this time.

I made sure my gas tank cap is in a very good condition, checked engine oil cap and dipstick.. make sure the gasket on those are in good condition as well.. I looked into the throttle body and saw some oil collecting there and thought about PCV valve (never changed).. too bad it's in an odd place and it was too difficult to get to from under the hood so I give up changing it myself... today while driving in a chill morning, I turn on the heat and discovered there is no heat coming out.. I turn off the compressor and kept fan speed at max and turned temperature to max, nothing seem to work. Any idea?