Dear fellows, please be patient to listen to my epic and help me with advice.
In 2008, I bought a certified used Honda Accord of 2005 in Iowa, USA. Everything was alright before I moved to Toronto in January, 2011. In the summer of that year, the Check Engine light (the manual calls it Emission Control light) came on. A private mechanic diagnosed a problem with the back O2 sensor and replaced it. In about a month, the lamp came on again. The mechanic scanned the car system and didn't find the code failure. However, the lamp would intermittently turn on and off. I took the car to one of the Toronto dealerships. The diagnosis was the same but I could not afford their price and took the car to a famous local car repair company. They also replaced the sensor but the lamp came on shortly and was coming on and off periodically. I had a chance to travel to Iowa and took the car to the initial dealership for repair since I had a plan with them for free fixing. The lamp was off after that for ten days but it came on again. The first mechanic checked the O2 and again, the computer showed "No fail code". He cleared off the code; however, the lamp came on again and since then, March 29, it has steadily been on. The code of that O2 is P0139.
What's happening and what would you recommend I do? Thank you, Ashot.

 

Is this a 4 or 6 cylinder? Was the replacement O2 sensor a Denso brand sensor? I assume the independent mechanics didn't have or use a scan tool capable of reading live data like air fuel ratio mix, etc. Did they check for other items such as exhaust leak, wiring, etc.?

 

[quote=redbull-1;276394]Is this a 4 or 6 cylinder? Was the replacement O2 sensor a Denso brand sensor? I assume the independent mechanics didn't have or use a scan tool capable of reading live data like air fuel ratio mix, etc. Did they check for other items such as exhaust leak, wiring, etc.?

Thanks for your efficient answer. Have no idea about the sensor's brand. All three repairers told me they had scanned everything. My Honda is 4 cylinder.