Volkswagen Repair: Jetta, th morning, collateral damage

Question
QUESTION: Hey Rick, I appreciate your willingness to help with cars you may not be familiar with. I have a '98 Jetta, 80,000 miles. I recently had some brake work done...and learned I have another problem I plan to take it back for soon and would like any opinions or thoughts you might have. The problem described by them after the brake work. The check engine light was on...and the code had something to do with the exhaust and something else I can not remember. They said there was oil in the air filter. I did not have time to leave it with them and now plan to take it back. Symptoms i notice myself that are really really pressing me to fix the problem are: In th morning when its cold...the engine starts ....it works some...but it starts and I am comfortable and confident. If I start later in the day after car has been sitting for some time and it has been warm outside, i have to prey....start start start please...the engine evnentually turns over...and then it takes a few minutes of huffing and puffing until i get the sense ok..now its working. After this...if i stop and restart....no problem. Any ideas or opinions?

Thanks

ANSWER: Barn,
 Sorry for the delay, I caught a bug and was down for a while.
I can't say for sure, but sounds like the O2 sensor needs replacement.  

Rick


---------- FOLLOW-UP ----------

QUESTION: Hey Rick...thanks for the reply and thoughts. I am asking out of ignorance...but 'sensor' to me suggests it just senses? But it seems like to me your thoughts imply that it senses and actually make adjustments based on whatever it is sensing?
Assume that is my problem...any collateral damage by running the car?
Hope ya feel better and thank you!

Answer
Barn,
 The O2 sensor is a critical part of all modern fuel injected engines.  It looks like a spark plug and is normally screwed into the exhaust pipe just before the catalytic converter (Cat), on some cars there is also a second one after the Cat.  What it does is measure the amount of oxygen remaining in the exhaust after engine combustion.  It sends a signal directed back to the ECU (computer)  to improve the fuel/air mixture until very little unburned hydro carbons remain.  Another words, it allows the ECU to dynamically "tune" the engine perfectly to the existing conditions almost instantaneously.  
 Now with this concept in mind, if the O2 sensor malfunctions, this can result in very bad things, from hard starts, badly running engine, terrible emissions, and possible damage of the Cat.  The good news is the ECU closely monitors the O2 sensor (as well as literally hundreds of other parameters) and if it detects problems, will illuminate the "Check Engine" light as a warning.

Rick