The other day, after a fresh oil change and brand new set of tires, I was driving my 2003 S-Type 4.2 on the interstate when the check engine light came on with the restricted performance notification. When I got to my destination, I checked under the hood to discover the air intake was loose from the throttle body (a reoccuring problem with that seal after mechanic visits). I reattached the hose tightly and then traveled to the nearest Advanced Auto Parts to get the error codes (P0171, P0174, and P1111) and clear the CEL. After this I was able to drive for several days without error so I thought I identified the issue. This morning, however, the CEL came on again and the air intake is still fully secured to the throttle body. Any ideas on what could be the problem? I was thinking the MAF sensor or an air leak somewhere. My car has 98k miles on it, but has been relatively error free since I bought it with 37k miles on it.

 

Even small leaks are very bad news. P1111 is a status code not an error - search on it but better still get JTIS (ebay workshop manual, about $10). P1071/4 are lean codes.

You need to find the current codes. To make it easier, get a cheap OBD tool (see previous threads).

Also, at the mileage your plugs are due to be changed now. Probably one or more coils, too.

 

P0171 = Bank 1 combustion too lean.
P0174 = Bank 2 combustion too lean.
P1111 = OBDII checks complete.

So yes, you could have an air leak. JTIS says that the cause could be from misfires, an air leak at the MAF sensor, the MAF sensor itself, a fuel filter issue, a fuel pump issue, and several other possibilities.

But you also need to be aware that the throttle bodies in these cars absolutely despise moisture. If even the slightest bit of moisture gets into them, the engine will go into "limp home mode" or restricted performance in a heartbeat.

Has it been raining? Had you recently washed your car? Did the shop who did your oil change clean the engine bay or the area around your throttle body with water? If the seal between your air intake and throttle body isn't waterproof, that's your likely culprit. Some folks seal that area with various silicone compounds or waterproof tape to ensure that all moisture stays out of there. Search this forum using "limp home mode" or "sealing throttle body" as your search phrases and you'll uncover a variety of threads that should help you....

 

thanks for the info, guys!

follow-up:
i cleaned my K&N filter and cleaned my MAF sensor which seems to have fixed the error. the CEL has disappeared and have had no problems the last couple of weeks.