I was thinking Mass Airflow Sensor. An unknown reading from that would cause mixture problems, but normally, you would expect a few other codes for this condition, such as P0170 Fuel Trim.
I have also heard of the post-cat O2 sensor causing pre-cat O2 sensor tests to fail, but normally the efficiency tests, not the limit tests.
I did a quick forum search on "E3" and came up with dozens of hits. Maybe you should look over those, as well as "E4."
It might be worth the expense to have more diagnostics done. A well-equipped shop could tell you the oxygen levels in the exhaust, read the various sensor levels, and quickly eliminate a lot of unknowns.