I have problem with my honda accord 2001 ex v6. It has been happening since six month ago. When the car run in highway about 20 mins, the check engine light go on, but I haven't seen any problem, and next day, after one night stay cool, the check engine light go off. I have bring the car to autozone for check. The result show that problem due to Oxigen sensor( Oxigen sensor low voltage). So, Do I have to replace the oxigen sensor. if so, where is the place that have reliable mechanic- I live in Lake Charles, Louisiana. Instead, my car has very bad gas mileage, about 17 MPG. what can I do to improve that.

 

Replace the O2 sensor yourself, its not difficult at all. There are threads to increase gas mileage all over H-T so use the search function to find those.

Here's a start.
https://honda-tech.com/search.php?searchid=13210645

 

You have two oxygen sensors. Normally it will tell you which bank is the issue..however it would be best to just replace both. You will need to replace only the "upstream" sensors...no need to replace the downstream one.