the service engine soon lite just came on on my 2001 3.8 stang with 102,000 on it. it has an exhaust leak on the passenger side at the manifold could it be serious

 

No, but it is VERY common. The A/C drips right there (bad design form Ford). They crack all the time.
Two options:
1) Pull it off, and replace it with one from a junkyard.
2) Pull it off, have it welded and put it back on.

Whatever you do, don't waste money on aftermarket headers - they don't do a thing for performance and will cost you way too much to be worth the trouble.

 

While LilRoush is correct in what he says, I wouldn’t do any of it before getting the codes read to find out what was causing the CEL to come on. Most auto parts stores will read the codes for free (call ahead of course). Make sure you get the codes, not just the description. Then do some googling & searching on this forum to see what the cause could be and possible fixes. Don’t guess. Fix it right the first time.

 

If he has a cracked manifold, why would he wait to see if it's wrong? Am I missing something?

If exhaust gas leaks out prior to being read by the O2 sensor, it's going to read a lean condition, which is going to dump in more fuel (actually creating a very rich condition), and trigger the light.

Fix the crack, mpg will improve and the Air/Fuel ratio will correct itself.

 

Your assuming that the crack is the source of the CEL. From the OP’s first post, the light just came on but he already knew about a crack in the manifold. So the crack has been there for at least a little while. So the crack may not be big enough to cause enough air to get into the exhaust to mess up the air/fuel mixture in order to trigger a (false) lean code. Not to mention there could also be more then one code.

So instead of guessing that the CEL is from the crack, get the codes read and fix whatever codes come up.

 

Hi guys: the manifold is not cracked the bolts came loose form the manifold and the exhaust pipe. I will tighten them up and see what happens.Thanks for the help.

 

Good point. I read it and assumed he was meaning the code was from the crack.