Need input! "Service Engine Soon"HELP!

[JD-S14]

Sup guys.

I need your help. I just bought a used 98 240sx SE this last Monday. Everything has been going just fine until today. The car is damn near immaculate in terms of cleanliness, so I was just putting loom around some of the hoses and lines to make the engine look even better. After I got done with that, I was getting ready to go home...and I wanted to try the "shiftkit" mod again that I read on this board (for the automatic cars). So I unplugged the resistor while the car was off. I turned the car on, walked away for a minute and when I came back, my mom was behind the wheel with a grin on her face like she wanted to take it for a test drive. She was reving the engine just a little bit, but nothing major. When I went over to tell her that the car was going to shift harder, I looked inside at the gauge display and noticed the dreaded "service engine soon" light. I checked over anything that I could have bumped when putting the wire loom on, I plugged the resistor back in, check the oil (ok), and turned the car off. The light still wont shut off. When I tried the resistor trick the other night, I had no problems with it. The only difference is that when I did it the first time, the car was still running while I unhooked and rehooked it back up. This time the car was off, I unhooked it....and I have a service engine soon light. Could this be the culprit, and if it is....what can I do to reset it? Unhook the battery for a while? I know with my Cavalier....if the car saw anything unusual (like when I left the AIT sensor out of the intake tube...it tripped a light, but all you had to do to clear it was to plug in the sensor and do a cold restart on the car 3 times. Then the car would reset itself. Worked like a charm.

Is there anything like that with the Nissans? Sorry if I seem like a twit, Im just trying to get used to a different car.

PS- Do these cars have any notorious problems that I should be looking/listening for? I swear after that light came on and I started to really listen to things, that I could hear something under the valve cover. No knocking or anything. But just a noise that I dont "think" I heard before. Maybe Im just hearing things now.

*worried to death that my new baby is broke*

Please help,Brad

[sixxdeuce]

first off, dont unplug that resistor, its not good for your transmission and will not make you car any faster, despite what it feels like. I tripped the engine light a few weeks ago because of the ait senor. I reset the ecu and it went away. the ecu is right next to where the passengers right foot would be. Do a search for "reseting ecu". There is also a link somewhere for a page you can look up the code it gives you

[sixxdeuce]

oh yeah, one common problem with all 240s is the timing chain guides wearing out and letting the chain wear holes in stuff, listen for a ticking at the front of the engine

[yellow_jacket]

Go to autozone and have them pull the code and shut the light off. It's free. Or find a friend with a scanner and have them do it.

[blackbeauty240]

if unpluging the resistor made the light come on then plugging it back in wont make it go off. its obd2 and even if you corrected the problem the code is still in memory and must be erased using a scanner.