Nissan Repair: check engine light, nissan frontier, oxygen sensor

Question
I have a 2000 Nissan Frontier V6 Auto trans which has the check engine light on about 3 years but it will turn off by itself after a week or so and it will be on again when the vehicle travel on a certain roadtrip up the hill (when the front of the vehicle raise up) the check engine light will
be on right away. I have replaced the fuel cap many times but didn't help. Any answers will be
appreciated. Thanks  

Answer
You need a code reader to be sure.  But, since you say it is not the gas cap it is probably the oxygen sensor.  There are two so you really need to get the codes checked.  Take it to Autozone as they will check the codes for free.  Check them out or go and buy your own code reader.  About $100 bucks for a code reader.  Since the dealer will charge you $100 just to read the codes it is worth getting one.