I have a 1990 240sx fastback. OBD 1 I believe. well the problem is I have ran the car for a while without O2 sensor because I have to go somewhere to get tools to change it. So the engine check light turned on when I drive. After that I install my new O2 sensor and unplugged the positive of the battery for 15min, I think that will reset the ECU. then I plugged the positive back but the engine check light is still on. do you guys know what's the problem and how to fix it?

 

I have that on mine too. Its on all the time.

 

well I think it is because you have OBD II which you need to have some tools to turn it off. but I think my OBD I can be turned off by take off the battery for 15min

 

did you check the ecu and get any other codes? My check engine light has been on forever 'cuz I haven't bothered to replace my oxygen sensor yet, but I know there's something wrong with my egr too 'cuz I get the code for it. I have a zenki, but I think it's obd 1... can't remember the exact procedure to reset the ecu, but I think it's in the fsm

 

Check the ECU for other codes that might be in there, then reset the ECU using the screw (not the "disconnect the battery way). If the check engine light stays on after that, you need a scanner to reset the light

 

up to '95 are all ODBI
as raine said, try using the little **** on the ecu itself.

i had a similar problem with my CEL.
code 41(iirc) for IAT sensor.
replaced sensor. i manually reset the ecu 5+ times and every time after 2 starts the CEL came back on.
took it to nissan dealership to check for faulty wiring.
they reset the ecu with consult and it hasnt come back on since.