Volvo: engine surge in 2000 S 80, electronic throttle control, oxygen sensor

Question
Could it possibly be an oxygen sensor?  It seems that the dealership had to replace this once before.  Thanks so much for all your help!
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The text above is a follow-up to ...

-----Question-----
Hi!

I have a 2000 S 80 that will surge when stopped.  It only does it when the ac is on.  Have already replaced the mass air intatke sensor.  That stopped the surge when the ac is not on.  Please help--I'm a single mom and can't afford to take it to my dealership!
-----Answer-----
Ramona,
There have been a lot of problems with the "Electronic Throttle Control Unit". That part in most cases is under warranty by Volvo. Surging has been one of the problems. Call you local dealer and see what options you have.

Answer
Ramona,
The O2 sensor will leave a fault code. Is the check engine light on? If not most likely it is not an O2 sensor. If the check engine light is on have those codes read and then you can more or less get a gereral direction to go in. Very many idle problems have been linked to the "Electronic Throttle Control".