Mazda 626 O2 dilemma

Hello all, I’ve been dealing with this O2 sensor issue on my 96 626 2.0 AT for a couple of years now. A little history: Symptoms = surging and waning at highway speed requiring deeper throttle pressure. This symptom will suddenly start or stop but gradually gets more frequent and last for longer periods.
My wife’s mechanic “cleaned it once for her for $100 and it fixed the prob for 2 years until last winter symptoms began again and increased in frequency and duration until I finally replaced the upper O2 sensor in the manifold. It appears there is another one in the header pipe below. Replacing the upper one fixed the prob again until recently the symptoms are beginning again.
I will try cleaning the upper one(since I know it comes out ) and see if any difference. I do not know if replacing the lower one will have effect.
Any observations or advice from you fine folks will be greatly appreciated.
Buzz

Ok an update, I got both sensors out, the newer upper one and the older lower one. Blew both out with compressed air and reinstalled. Seemed better for couple hundred miles, then began symptoms of waning power and a little herky jerky. Usually these symptpms clear up after 10 minutes or so but it gets longer and more frequent. It seems the sensor(s) are telling the car to run lean as mpg does not go down. Thanks for any thoughts!!

hmm... have you considered changing your fuel filter? it cheap and its a maintenance item, it should be done anyway. but failing that you may want to look at your mass airflow sensor. its on the intake hose at your air filter box. those can get dirty and cause loss of power symptoms. you can clean it with some spray carburetor cleaner. dont try to pick at it though... the sensor leads are delicate.

Thanks mazdatech177!! due to your advice it dawned on me that I had not even changed the air filter in about 40,000 miles...duh!....your advice about the MAF really clicked because I was certain the car was running lean when malfunctioning. Anyway, a new air cleaner and spray of carb cleaner in the MAF and no more problem. Hope it lasts. I'm beginning to think it was just the dirty air filter. I will change the fuel filter on gp as you suggested.

thanks again!!

Buzz

no problem at all. i hope it stays fixed for you

It has been 6 months and no problems. The car is running well and getting great mileage 30+ in warm weather. The car has 228,000 on it. I just replaced the Air filter again. I drive about 750 miles a week so I am pumping a lot of air and fuel through the filters.

I now see I have an anti-freeze leak, looks like the water pump. It is just seeping a little right now. I will monitor and hope it lasts to warmer weather comes to MN.

Meanwhile I'll search the forums for some insight to the job.

Thanks to all!!