I checked, looked, tightened and searched but I couldn't identify the problem. It wasn't until I removed the intake manifolds and put new gaskets on that the code went away. With the leak the car would idle a little rough, not too bad, and the idle speed was higher than normal. Just tightening the bolts didn't help. I had to change the gaskets. Good luck, I hope you find another cause but I think you have a gasket leak.

Les