engine check light comes on

I have '90 Honda Civic. The engine check light comes on when the engine goes at high speed or up a hill. Is it the O2 sensor? Now I'm having trouble starting it when it's hot. Is it hard to replace the sensor?
Thanks,
Z

I posted this earlier in the wrong forum thread:nono:

you need to findout the code its putting out... to do that pull back the passanger side carpet...where your feet go. and you will see the ecu..and a window on it...turn the key to the run position and count the blinks...it will stop for a min and start over or show another code.
count it and tell us what you get...
i will bet your starting problem is from the main relay...it is a relay that turns on your fuel pump... the orignials go bad in high heat...you have to get it from the dealer they were like 60 bux when i got mine about 2 years ago. its kinda hard to get to but its around the same spot as your hood latch
just up and closer to the side, it says main relay on it.

Thanks for your reply. I'll check it out.
Z