service engine soon lamp lit

Hi metro men, i'm back! Maybe someone can steer me in the right direction. 1997 geo metro, 1.3 4 cyl. auto trans. Replaced head gaskett, and cracked exhaust manifold. starts, runs, handles excellent.
Problem is i have the service engine soon lamp on, not the check engine lamp. I disconnected the battery and the lamp went out for about one day, its now back on. I seem to of heard this would be emissions related, like o2 sensor or something? I did not reuse my o2 sensor, i used the one that was in the replacement manifold i installed. Any suggestions? before all this engine work there never were any codes, or lamps lit. Thank you all very much for any help, or advice. Everyones been GREAT in the past. Wonderfull site!! Thanks.

You need to scan with an OBDII tool to find the stored code(s) that tripped the SES light.

A battery disconnect is essentially a 'reset' that clears the code(s), and after you've driven/run through a series of cycles
[see http://www.obdii.com/drivecycle.html], you'll give the ECU enough time to gather all the data it uses to assess if the SES should be illuminated.

You can find out more about OBDII at http://www.obdii.com/

My '96 3-cyl. will throw a P0420 every so often, but it burns cleanly enough and gets decent mpg (over 40).

Mike, thanks man! I kinda,sorta,momentarily thought it would involve a scan proceedure, and not a "quick-fix". I just really didnt know. Great link you provided, maybe someday i can help others as a pay back. thanks.

my engine lite is on and the OBD11 read the intake air tempsensor was bad but i cannot find it. can some help me on this or direct me to a picture of it on an engine. thanks

my engine lite is on and the OBD11 read the intake air tempsensor was bad but i cannot find it. can some help me on this or direct me to a picture of it on an engine. thanks
Spuder, the intake air sensor (IAT) is located in the air cleaner assembly. the ROUND housing portion that the air filter sits in. On the drivers side, about 3/4 of the way back. Will have an electrical connector attached to it. The part will be about 3 inches long when you pull it out of the air cleaner housing, and is black plastic in constructin.
almost cant miss this one as it is the only one attached into that side of the air cleaner housing. Hope this helps and i didnt make things confusing, just trying to help. Good luck.

thanks a lot blownpiston, i got it. by the way would that mess with my milage.

thanks a lot blownpiston, i got it. by the way would that mess with my milage.
Spudder, To test the IAT sensor,
1. Remove the sensor 2. Place the tip of the sensor in water.
3. begin to heat the water while taking readings of the resistance.
4.If the resistance does not decrease as the temperature goes up, replace the sensor. HOWEVER, if the sensor checks out o.k., check the circuit from the sensor back to the pcm and to ground for any voltage drops. IAT sensor is used to measure the temperature of the intake air about to be used by the engine. The sensor uses resistance to turn temperature into a voltage that the pcm can use. the sensor signal is used through all the phases of engine operation from cold start to closed loop operation to tune fuel delivery. so, yes this part could affect your gas mileage. also a faulty IAT circuit as mentioned. all this from chilton book, i assume persently you need one so reason for this long post. sorry. But please let us all know what you find to fix the problem.

well i got a new sensor and reset the computer by disconnecting the battery cables. after driving five miles the service engine light was back on. i haven't checked the circut as i just don't have the time to play with it. it does run good but i am only getting 30 mpg. i sure wish it was better.
thanks for the help