My '99 Plymouth Voyager is getting about 3 miles per gallon; used 1/2 tank
to drive to Detroit and back to Ann Arbor (about 60 minutes of driving).
Notice needle going way down on short trips as well.

No smell of gas, tank doesn't empty when car's not running, exhaust doesn't
smell overly gassy.  Replaced thermostat, air filter, and O2 sensor about 2
months ago.  Engine sounds fine, seems to run exactly as it has for 6 years,
only 70,000 miles on it.

Does anyone have any idea of what could cause this tremendous use of gas,
with all of the above?  My only guess was that the O2 sensor was put in
wrong, or was the wrong type, but a mechanic told me over the phone that I'd
get trouble codes or something if the O2 sensor was the wrong type, or not
working correctly.

This is a really serious issue, and I was hoping to get some advice before
paying $100 to have the thing hooked up to the computer.  I mean, w/ no
Service Engine light, that means there's no trouble codes, right?  So
wouldn't the big computer say the same thing?

Any advice whatsoever would be greatly appreciated, and thanks for reading.