99SH

I changed the primary O2 sensor in my gf's MR2 at 115k and saw a significant improvement in MPGs.

Slowcrash_101

When I bought my 02 it had 113k and at around 120k the oil started smelling like gas, so when I did my clutch I replaced the O2 sensor since I had to remove it anyway. Car idled a lot better, oil stopped smelling like gas, and mpg improved. Then again PO wasn't the most meticulous about car maintenance.

That probably had a lot to do with it, if you take care of the car then I'm sure the O2 will last longer. Things like not using the car for 6 months straight with less than a quarter tank probably didn't help. Most O2 sensors, start getting lazy at around 120-140k miles but it's just a general rule of thumb.

Slowcrash_101

A lazy o2 sensor will make the ecm think the car is running lean at all times and will pour more gas into the mix. If you have a cheap obd scanner check your o2 voltage, a bad o2 will have voltage change very slowly.

You always get some blowby even if your engine has perfect compression.

If the fuel trims don't go more than + or - 7 long trim it also won't trigger a code despite heavy adjustment for short trim.

Car Analogy

I changed my primary O2 sensor at around 115k. My mpg had fallen, and I tried this to see if it would improve. It did. Glad I did it.

Sent from my SM-G920P using IB AutoGroup

Billman250

Aside from MPG, another symptom to note is if the chrome tailpipe tips turn black frequently.

HarryD

My tailpipe tips change from chrome to black practically overnight.

So changing the primary o2 is a good idea?

Is leaving the secondary (the one in the cat?) alone OK since it just measures the difference between the primary and itself and throws a code if there's a big discrepancy?

zeroptzero

^ yeah don't bother with the secondary sensor until it dies, and it has no effect on your current concerns.

solitarycheese

I changed mine at 104k. Engine feels like it idles and revs smoother, and my fuel economy has improved about 1MPG. I was experiencing high negative fuel trims, soot on my tailpipes, and a slight smell of fuel after idling for a while.

I think it would have been "fine" to wait for the sensor to fail entirely, but I like things to be in tip-top shape. It was $93 from Amazon and took an hour or two in my garage.

windhund116

"It was $93 from Amazon and took an hour or two in my garage."

This is R&R'ing the front O2 sensor (aka: air/fuel ratio sensor on AP2's)? Must be a trick to this. Took me 3-4 hours to remove and move stuff ---plus a LOT of cussing --- to get to and replace this sensor. Rear O2 sensor is a 1/2-hour job, no sweat.

HarryD

Updates:

• The new Denso sensor from Amazon is DIFFERENT from the one I pulled off the car
• Old sensor: not sure if original from factory or if previous owner replaced it--no brand name on it, only markings "OHD ::: 508-H1" -- google says this is an NTK sensor.
• Kind of bummed I didn't get NTK, hopefully this Denso will be fine?
• New sensor: has 3-4x more "holes" (picture below) -- not sure if this is good or bad!
• Followed the DIY: https://www.s2ki.com/s2000/topic/102...placement-diy/
• The plastic clips were the "hardest" part to remove -- be sure to rent the o2 sensor socket set from Autozone! Took ~1 hour incl. jacking up on all 4 jackstands
• Went for a drive yesterday and today -- no more soot out of the exhaust [yet!], only dripping water [yay!]. Will report back in a month if it changes.
• No CEL [yet]

Thanks everyone!