Bottom line here is what I would do:
--Go to Autozone and have them read the engine computer for
any stored codes. Report back.
--Change out coolant using a reputable shop. Use Honda OEM
coolant only. Make sure the coolant is topped off in the
--Change out auto tranny fluid using Honda OEM fluid only.
Then I would be looking at checking the idle RPM, the EACV
and the fast idle valve pictured at
More detailed responses to your other queries follow.
True regarding what the owner's manual says. My
understanding is Honda recommends 5W-20 oil to improve
overall fuel mileage for its fleet, and so conform with
federal fuel economy requirements. It is still okay to use
5W-30, and in fact Honda is not allowed to void any warranty
on the car if a person does use the 30 weight, despite what
the owner's manual says. See
But 5W-30 is a heavier viscosity than the 20. For Chicago
winters, I would have the 20 weight. On the third hand,
could the 30 weight oil be contributing to this
sluggishness? A tiny bit not likely to be noticeable around
25 degrees F.
I do not know how much you work with cars, but the above
sounds like overanalysis by a relative layperson. It's a
nice layperson's theory but highly unlikely. And no, there
is no such indication.
Honda automatic trannies circa 2003 are known to have
problems unless one uses Honda's OEM tranny fluid. I would
change this out, if only for peace of mind on such an
expensive part (the tranny).
On a 2003 Civic, yes, you should get a CEL if the O2 sensor
is faulty. But unless this car has been abused, I do not
expect its O2 sensor to fail at only 68k miles.
Furthermore, the engine control computer ignores signals
from the O2 sensor until after warmup.
Do you have an owner's manual? If so, look up the
maintenance required light in the index in the back. You
will find instructions on how to reset it. Walmart did not
do it because it is not their yob (sic). Walmart, Jiffy Lube
et al. do not deal with this much specialization. If you do
not have the owner's manual, then go to site
https://techinfo.honda.com/rjanisis/logon.asp . On the left,
click on "Owner's Manuals." Follow the pointers. Once you
arrive at the pdf file, it may appear that only one page has
come up. In fact, page numbers that are highlighted within
the pdf file are links on which you can click to take you to
the page. Also, in the upper right hand corner of any white
page look for an arrow. Click the right arrow to get to the
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