Mitsubishi Repair: running lean, mitsubishi galant, 2002 mitsubishi galant

Question
I have a 2002 Mitsubishi Galant 4spd automatic. Recently the check engine light came on, and it reported the engine was running lean. This happened right after I changed the air filter which was filthy, so my first thought was that the fuel filter must also be in bad shape. I looked all along the fuel line and eventually found that the fuel filter is part of the pump assembly, and not meant to be serviced as usual. The next thing that I thought of was the O2 sensors, but the dealership said they need to charge me $300 for a diagnostic check first... then the cost of parts, then the cost of labor which is always obscene. I guess my real question is, where is the intake o2 sensor? I found the exhaust sensor easy enough, but cannot locate the intake sensor (I was told by mitsu there are 2 sensors, in and out on this model). Also, nobody ive talked to will give me a rundown of what else could cause my car to run lean... I understand there are several things but any explanation you could give to elaborate a bit would be appreciated. I just bought this car about 6 weeks ago, and im seriously dissappointed that this amount of work was required after paying too much in the first place.

Answer
Chris,
Your O2 sensors are most likely located on your exhaust pipe under your car.  One is located before the catalytic converter and the other is located right after it.  You can usually see them pretty easily because they are the only object with wires coming off your exhaust pipes.

These are the upstream and downstream sensors.  The odds that these are bad are pretty slim unless your car was in an accident.  And the fact that your check engine light (CEL) came on right after installing a new air filter would lead me to believe the problem could still be related to your air filter.  Check all your hose connections and make sure the filter element is properly seated inside the air box.  Check all eletrical harnesses to be sure they are making good contact, and that you did not bend any pins on the eletrical harness if you removed it.  When your CEL is telling you its a lean mixture, many times that just means there is too much unmeasured air entering your engine.  That is usually caused by a vacuum leak in your intake system.  So check all your hoses and your air filter again.  Once you've double checked everything and everything looks good, remove your negative battery cable for about a minute and reattache it.  This will reset your CEL when you start your car up again.  If the CEL stays on even after resetting it, you should have a mechanic take a look at it.  Though it would not hurt to find a local mechanic other than the dealership to look at the problem.
Good luck!