The O2 sensors are generally not on a schedule these days. They are
normally changed when indicated by a check engine light & code readout
or when increased exhaust emissions indicate that the sensor might be
getting slow to respond.
That said, treating it as a maintenance item at 100k miles isn't a bad
idea. You are concerned about the first sensor before the catalytic
converter, not the second one after the converter.
If you do change it, stay away from the "universal" type which is
spliced into the line. You really want to use one with a correct set of
wiring and connector. Denso would be my choice on a Honda.
rockauto.com and others generally have them.
O2 sensors need to be scoped to be sure if they are 'tired'. OBD II
doesn't care too much if they are lively, so long as they still cycle.
Don't forget to do the PCV as well. It often gets overlooked, and it can
lead to the demise of your CAT if it allows too much oil vapor to be
sucked out of the engine.
It is a shame you are spouting bad information about OBDII. OBDII systems
monitor rise and fall time of the O2 sensor as well as voltage levels and
current levels. If any aspect of the O2 sensor goes bad the computer will
tell you. The original poster here apparently has no idea of how to
determine gas mileage. He needs to measure the mileage as well as consider
his driving habits and the type of fuel he is using. Fuel formulas change
this time of year which will cause lower mileage as alcohol is added. You
can start throwing parts at a perceived problem but you are wasting your
Just repeating all I learned (off the Internet) when my CAT went bad.
Many sites said that OBD II looks to be sure the sensor is cycling, but
isn't too picky if it cycles FAST ENOUGH all the time. As sensors age
they slow down, but OBD II is said to be way too tolerant (and it has to
be due to several circumstances), and, if you want to know _for sure_,
you have to scope them.
I don't think this is 'bad information'. If a person wants peak fuel
economy, you can't depend on OBD II to be the total answer.
This has been kicked around here at length before. I am still not convinced
one way or the other, but I tend to agree with you. There have been a lot of
OBDII cars that clearly benefitted from O2 (#1) sensor replacement even
though OBDII didn't complain. Of course, the picture is muddied by the
uncertainty as to whether a particular sensor is degraded in the first
place. If a perfectly good sensor is replaced, no improvement will be seen.
If a marginal one that is still working "well enough" is replaced,
improvements in fuel efficiency and drivability may be seen... but that
conflicts with the results from replacing the good one.
If I'm not mistaken, OBDII only requires emissions related monitoring. As
long as the front sensor doesn't drive the mixture so wild that the cat
can't clean it up, OBDII really doesn't care.
OBDII does not always catch bad O2 sensors. I just went through this on
my '96 OBDII equipped Volvo. The car's computer thought everything was
fine. NOX readings said otherwise. A new OE oxygen sensor fixed the
problem and immediately brought NOX readings back to what they had been
four years earlier at one of that car's prior emissions tests.
As the others said, there is no time/mileage spec on the O2
sensor. It is early for yours to be working poorly. I'd do
basic tuneup items first and then see how your car performs
for a month.
Heed to the letter the gas mileage tips for Hondas given at
the site below, and your mileage could improve
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