Check Engine Light On?

Engine Mechanical problem
2000 Ford Taurus Front Wheel Drive 54000 miles

my empty light came on so I stopped and put half a tank of gas in my car. The gas came from a station I have used before. And about 5 minutes down teh road my check engine light came on. I have bought gas before & had teh lite come on. I go and put a better grade of gas in and teh lite goes out. Do you think this is teh case with yesterday. I also had my oil change done after my lite had came on. They said I need a new airfilter. I get one tomorrow. Could that have made teh lite come on? Nikki

I have a taurs Se year 2000. I took the car to a mechanic because my car started stalling while driving in traffic. They checked car and said there is 3 misfires. They flushed the car becuase they felt that there must be carbon build up in the fuel system. Car showed up a check engine light on. I took it back and they said that it being the car has over 100,000 miles I need to change the struts and possibly coils. I only had struts changed because that is what the manuel had said that at 100,000 miles I should change them. The cars engine light is still on and now my mechanic is telling me that there is a problem with the intake and this is the reason it still stalls. What do I do could this be in association with my engine light being on?

My 2000 ford taurus idle's up and down when I come to a stop. When it idle's down, it acts like it wants to cut off but it doesn't. It also has a miss to it when it idle's down. What could that possibly be?

I have a 2000 Taurus wagon with the 3.0 DOHC V6 Duratec engine. I bought it used last year and It has 138,000 miles on it.
About a month ago, the transmission started giving me problems by not shifting when the engine was cold. I took it to a local transmission shop and it was diagnosed to be the speed sensor, which was replaced for $350. I thought this was a bit steep, but I paid the bill and was on my way.
The next day, I was on my way to work, when the 'Check Engine" icon lit up. This had never happened since I bought the car 14,000 miles ago, so I went back the the repair shop, thinking maybe they neglected to connect everything back.
The mechanic ran the diagnostics and the codes P0172 & P0175 [System Too Rich (Banks 1&2)] showed up. He thought it was my O2 sensors, but he didn't see anything physically amiss. He reset the codes and said it could be a glitch in the system and if it came back, he'd investigate further. By the way, the original problem was corrected - it shifts normally now.
A couple days later, the 'Check Engine' icon came on again, so I decided to take it in for a thorough diagnostic checkup at a reputable engine specialist in my town. He ran it through the diagnostics and reported that the P0172 code popped up, further explaining that the O2 sensor was switching slowly, causing the icon to be tripped.
He said that it would cost between $300 and $600 to replace the sensors and that his advise was to just ignore the 'Check Engine' icon, that it wouldn't be detrimental to my engine. He did suggest running a couple tanks of gas with injector cleaner in it, he reset the code, & I was on my way.
Well, about three days later, the dreaded icon came on again! Now my questions (finally!).

1) Do you think it could be just a coincidence that the icon illuminated right after trans work was completed or could the trans shop have caused it? They stated that they dropped the trans for the speed sensor repair, but didn't disconnect the exhaust.
2) Why would the icon not come back on immediately? As I stated, it would stay off for a couple days before reappearing.
3) Is the price I received ($300-600) reasonable to replace the sensor(s)? How many sensors are we talking about, which ones should be changed, and where exacxtly are they located? Can a mechanically-inclined person (me) change it/them or are there special tools/skills needed to complete the repair?
4) Or should I just take the last mechanic's advise and learn to live with a constantly lit 'Check Engine' icon?

One last note - the engine's performance doesn't seem to be affected by the icon being lit, but I did read that when it is lit, the engine's computer runs a 'limp program' that decreases gas mileage and increases emissions. Because of this, when the icon reappears every couple days, I disconnect the battery for a few minutes, which resets the code and the icon disappears for 2-3 days.

Sorry for being so lengthy, but I figure too much info is better than not enough! Edit at will! And thanks.

Paul, I had said that the idiot light came on the day after the transmission shop completed the repair, not a week later, but anyway, I inspected and cleaned the Mass Air Flow sensor as you suggested and reset the CEL 2 days ago - the icon came back on today. Now what? Should I replace the MAF sensor? In reference to my question #3:

Is the price estimate I received ($300-600) reasonable to replace the O2 sensors? How many sensors are we talking about, which ones should be changed, and where exacxtly are they located? Can a mechanically-inclined person (me) change it/them or are there special tools/skills needed to complete the repair?

Thanks.