Last November I bought a 99 Maxima SE 5-speed with 140,000 miles for my 16 yo son to be his first car. When I bought the car it was throwing a CEL code for the downstream O2 sensor. I soon replaced the faulty downstream O2 sensor and started waiting for a sale to pick up two additional upstream sensors. I figured the old sensors were factory OEM and should be replaced at the car's mileage.

After school was out in May, he started driving the car a good distance each day to his summer job. He had been complaining about poor gas mileage and the cost of premium fuel. I thought the complaints were based a fuel cost comparison with his buddy's 4-cyl Honda. So based on the 4-cyl (regular) to 6-cyl (premium) fuel cost comparison, I didn't pay much attention to (dismissed) the complaint and figured it might be a good thing to slow down the mileage that comes with "going places" during summer school break.

Last week the radiator had to be replaced. So while replacing the radiator, I decided to go ahead and replace the upstream front and rear O2 sensors. By this time I had used some sale coupons to save a few bucks on new NTK sensors. Please note the car was NOT throwing any CEL codes with the old upstream O2 sensors. This was a preventative maintenance operation based on the car's mileage with understanding the original O2 sensors should have been replaced at 100,000 miles. As soon as I finished replacing the O2 sensors and checking out the cooling system from the radiator replacement I took the car for a test drive. I could tell an immediate performance improvement. It seemed to loose some of the jerking that was happening in 1st and 2nd gear. So I put away my tools and called it a day.

Three days later I ask my son how the Max drives with the new O2 sensors. His response: IT HAS MORE POWER and IT USES LESS GAS. So I want to use the money I'm saving on gas to buy a new Front Strut Tower Bar(FSTB). Well that's ok with me.

Since I didn't measure the bad mileage with the old sensors, I can't be scientific and post the mileage in MPG numbers. However, I guess from my son's perspective, the benefit to his pocketbook in (one week) fuel cost saving resulted in a new FSTB (~$30). Yep.. get more FSTB (and other performance accessories) by saving fuel with O2 sensor preventative maintenance. Note: He keeps the car rolling when he is not working or sleeping so he is racking up the miles on it like most kids do at his age.

NTK/NGK Part numbers for a non-CA model:

Quantity (2) NTK24525 - Upstream Right or Upstream Left,
Quantity (1) NTK24630- Downstream

Approximate NTK/NGK cost using internet sale coupons ~$200 shipped

Failing O2 sensors tend to fail on the 'lean' end (closer to 0.2V), thus ECU commands more fuel thru the injectors. Aside from low MPG, a 'rich' mixture used for extended period can shorten the lifespan of the catalytic converter(s).

Since the car is old you might want to make sure there are no small holes in the exhaust
system. I found a crack in one of the pipe seams on mine and plugged it up with some
thing similiar to jb weld. Air enterering before the catalytic converter could make
the O2 sensors generate bad data to the ECU for engine adjustments.

I did the same thing as you and replaced mine in the 125-150K mileage range.
They were starting to throw codes too-including the knock sensor when nothing is wrong with it.
The car did have more power.

You might check the front end before buying fancy struts to see if parts are needed
I recently spent about $400 on Moog control arms, connecting rods, and inner and outer
tie rods and had the car aligned at Midas. Now it rides great- the struts were fine

BTW your engine looks perfect!

NutriaforBreakfast wrote:Since the car is old you might want to make sure there are no small holes in the exhaust
system. I found a crack in one of the pipe seams on mine and plugged it up with some
thing similiar to jb weld. Air enterering before the catalytic converter could make
the O2 sensors generate bad data to the ECU for engine adjustments.

I did the same thing as you and replaced mine in the 125-150K mileage range.
They were starting to throw codes too-including the knock sensor when nothing is wrong with it.
The car did have more power.

You might check the front end before buying fancy struts to see if parts are needed
I recently spent about $400 on Moog control arms, connecting rods, and inner and outer
tie rods and had the car aligned at Midas. Now it rides great- the struts were fine

BTW your engine looks perfect!

Thanks NutriaforBreakfast! BTW we were also getting random KS codes before the O2 sensor upgrade. There must be a connection.