2004 4Runner SR5 engine issues

I recently experienced what I thought was a found spark plug in cylinder #6. I had the plugs replaced and it showed the 0306 code after driving it 60 miles and then letting it cool down. I took it back to the shop and had a coil and fuel injector replaced. It is still coming up with the 0306 code after being reset and allowed to cool down. Any ideas on what may be causing this issue? I have dumped over $800 in a week alone and cannot figure out the issue at hand. Any suggestions or ideas would be greatly appreciated.

 

Quite likely the cause is a blown head gasket. Very common prior to early 2006 when the head was slightly redesigned.

 

I had the valve springs all checked and none of them had any stress fractures or any abnormalities. I hope it is just a head gasket. I only have 134,500 miles on it and have never had any serious issues with it. Regular maintenance is about all I have done on it. Great durable vehicle overall.