Volkswagen: Check engine light, vacuum hoses, vw cabrio

Question
1999 VW Cabrio, 34,500 original miles.

My check engine light came on last year.  I brought it to the dealer (long story short) and fixing it was cost-prohibitive. I did make some of the recommended changes--change spark plugs, vacuum hoses, air & fuel filter.

I needed an inspection and had to bring it in for the repairs earlier this month.  They replaced some sort of pump ($655 for the part alone) and said that I needed to drive my car for "two complete cycles for the sensor to reset."  I tried to do this, but my light is now back on and they have my car again.  The diagnostic reading was PO411.

I have to have my car.  What questions / comments/ recourse do I have to help me accomplish this goal?  Am I not asking the right questions of my service advisor?  I was told that replacing this pump would fix the problem, but it hasn't.

Answer
to start with, if you were told that this pump would fix the problem and it hasn't i would be making sure it was fixed without any further charge. or possibly contacting volkswagon regional headquarters and making a complaint. i will look into fault code po411 to find what are the likely causes.