check engine light on jdm b18c

See if you can figure this out. When I start my car there is no check engine light. All is good. I drive the car and when temperature reaches the normal operating temperature, which is around half way up the temp gauge, the check engine light comes on. There is no way to check for a code because as soon as I turn the car off and back on, the check engine light is gone. until it warms up again and the light comes up again. Somebody told me it was probably some wire that was connected wrong during the install. I dont think its a big deal either but I would like a check engine free jdm swap. If anyone knows how this can be fixed please let me know. Thanks a lot!!

You should still be able to pull the code.