02 life span
 

How long do 02 sensors generally last?
I've been fighting declining mpg since last summer and am finally at the point where I believe it is that gas quality has taking a BIG hit, or that my 02 is going south on my 4 yr old sweetie.

In my experience O2 sensors will usually last 10+ years. Usually if an O2 sensor goes bad you'll see a huge drop in FE. When the O2 sensor went bad in my '88 Escort Pony mileage dropped from 40+ MPG to about 25 MPG. Is your check engine light on? If it's a bad O2 sensor, there should be a code stored in the ECU. If the CEL is on have the codes checked, that should lead you in the right direction for finding your problem. Looking at your fuel log it looks like the FE hit started in late summer/early fall, if that's the case there are a couple other things that could be contributing to the problem, winter blend gas and cold weather, both can cause a decline in FE.

When it started my mpg had dropped to the low 40's, the last tank was high 34s.

Swapped the plugs at 45k and have 52k now, the original set looked good but the 2nd set was quite a bit darker around the base ring. I have been noticing pinging and worsening drivability, and it's getting more so. No CEL though. Tried various grades of fuel from differing stations as well.

It's got me so ill I have not updated my fuel log in a while.

Going from 44 mpg in summer to 38 mpg in winter is pretty normal.

I was getting around 50 mpg in summer with the Metro, and now I'm in the low 40s.

Why not put the old plugs back in and see what the result is? The pinging bothers me.

What prompted you to change the plugs?

Run some Chevron Techron through it. I got 6 pint bottles at Costco for $19.99. Kia calls for it in their direct injection engines.

regards
mech

I ran the Redline S1 two tanks in a row. The last 2 winters have netted 40 mpg, or very close. No reason for 34 mpg! I'm not one to run plugs a second time due to the crush washer being used already. It might be OK but i wont do it.

I have always heard the 100k rule of thumb for o2 lifespan, but to my mind, if it isn't throwing your FE off and your fuel trims are ok, I see no reason to change them.

FE is always such a tricky beast. There could be 100 reasons why your FE is low, o2 sensors among them.