The Check Engine Light yet again?

Ok, so I take my 996 in last week because the overflow tank was leaking and I had the exciting "Check engine" light go on. So, they replaced the tank, and it turns out that one of the O2 Sensors was out as well. They replaced that as well. They did the standard stuff, ie..re-fill the coolant fluid, check pressure etc. Everything looks great, I pick up my ride with my wallet a tad bit lighter, but nothing too serious. As I leave the Garage, the car is running very strong and I am very happy for the work done. I drive home and don't touch the car the next day, the following day I was going out with my GF and I was happy to have my car back, so we took that and it was a very cold night, but dry. (I will not usually drive it when it is wet or snowing.) All of a sudden, the "CEL" Pops on? My first thought was to dbl chk the work they did, and everything looked fine. (This garage had worked on the car before I owned it, as well as another BMW I still own, so they pretty much have my trust.) However, I have to admit I was very confused that night? A few days go by and I am finally able to take it in again, today.......They check the OBD II reading and they got some code that read something vague like "Fluid, air mixture off" or something like that? I will post the exact statement tomorrow as the guys at the garage have already gone home today! Anyways, while I was there I talked with the head guy and he says right now since the car is running fantastic, he doesn't see any problems, but will look into the code that he got to see if he can determine anything new and will call me tomorrow? He says to drive the car home, and let him know if there is any change in the car or if the light comes back on. When I called he had just left, but I will call tomorrow. So he shuts off the "CEL" off, and I am driving home, I am taking it fairly easy as I don't want to see that light again!! (I will note that there are a few puddles around because the streets in Michigan are taken care of so well. (We have a perfect Government here, so why wouldn't they be in perfect condition? ) Sorry I couldn't resist. The roads here are worse then when I drove the Alaskan Hwy back in 1988 after College.) So, I stop at a store for a soda, jump back in and pull out of the P-Lot, and there is a light about 75 feet away. I look down to check to see if the light is still off, and all of a sudden the damn thing "Pops" back on while I am looking at it, as if to spite me or something? I was just sitting at the light waiting for it to change, no rev, no nothing, just regular idle? So, I am happy at this point! because all I did was pull out of a P-Lot and pull up 75 feet and it goes off with out any provocation? I apologize for the length of this, and will work at keeping things shorter, but my real question here is this.... Has this ever happened to anyone before? Does it sound familiar to anyone? As I said, I will post the exact code and info tomorrow, because I know how much more sense it will make for everyone, but since I have the night to sit here and brew, and search for some kind of clue, I can only hope that someone has come across something like this before? (As you can likely tell, I am not the most patient man? I try, but when one of my toys is broke, I get very anxious as to how to fix it? (I cannot imagine that most P-Car owners are very patient people, but I am just guessing?) Thanks for any potential input ahead of time. ps. it is a 1999 996 FYI Just clicked 80200 miles. Just a side note: I went in the back of the shops yard and they have a 1969 911, it doesn't run, but I thought if I could help anyone that is searching for parts for a project car, maybe I could help them on this particular car? If you are looking for something for a 1969 911, just pm me and I will help as much as I can? ie..pics, numbers, contact info etc?

I would buy an OBD II reader for $80 and read the codes yourself. There is a lot of information available here once the code is known.

Check engine lights are not uncommon, however, as mentioned, a code reader and a specific code would go a long way in determining your specific issue. Not flashing is ok to drive, but you should know why it keeps returning.

A constant lit light means emmission related fault. I'd check catalytic sensor wire and sensor connections to the cats. A flashing light means combustion fault. Since I gave you the extra info of a flashing light I'm charging you double.

Check something easy - I had a similar problem after dealer service, and my air filter was installed incorrectly. That set off the CEL - quick correction and all was good. Hope that works for you.

You need your own code reader. Cheap$80-$90 bucks. You then can read your own code and erase it. My first year of ownership and my car was throwing codes every other week. My dealer is 90 miles away.Drove me nuts!!! At least you can have peace of mind knowing if it's an important code or not and you can erase it and get rid of that annoying yellow light on the dash.It will pay for itself after reading one code. Get one post the code here, then we can really help you.

i'd certainly look at the O2 sensor again, but once you get the code you can be sure. also if you're in the SE michigan area, you should check out my indie, it's always nice to get a 2nd opinion.

I read the whole thing.......

Can you buy a code reader at all the usual spots?

I read the whole thing too, and can confirm the roads in Michigan suck.

If the code reads something about fuel/air mixture, I'd check the mass air sensor.

I had the same problem on my '03 w/50,000 miles. I put in a new Mass Airflow Sensor (MAF) and that did the trick. It is on top of the airbox on the engine side, the thing with the wires going to it. Try to clean it.......It takes a special "safety torx" head for a screw driver, I bought a whole set of safety screw heads at Harbor Frieght.....$4 or something. Get yerself a OBD reader, too. ($56) You can find a 20% off coupon on their website.....no affiliation. Now start the car, slip in the plug of the code reader, down by your left knee on the lower part of the dash....wait for it to "talk to the car".....read codes.....clear the code. I kept coming up with (I think) P1126 and P1137, which are pre-catalytic converter O2 sensors. I cleaned them, I replaced one and still got the CEL just after or just before the water temp came up to 185 degrees. Sometimes. Take the Mass Air-Flo sensor out of the air box, clean it with MAF cleaner....O'Reillys Reset codes Adopt a wait & see Attitude Wait-you have a '99, I have a MAF I got from Ed, (doesn't fit my car) but he has a 99............so I think it will fit yours, I paid $100, if you want, I'll send it to you, if it works and solves your problem, send me $100, if not, send 'er back What a deal. No, I'm serious. I got your PM but couldn't replay Stu

^^ Get a T20 security torx bit, get CRC MAS/MAF cleaner, and try it.

Handle the MAF very carefully don't touch the fragile wires you see in the sensor!! Flood it with 1/3 can of electronic cleaner or MAF cleaner from the autoparts store. Take it outside to do this as the paint will come off your garage floor from the drips. Or the fumes may kill you. it cured my car of throwing O2 sensor codes.