Toyota Repair: MAF sensor on 2000 Corolla

Question
QUESTION: I have a 2000 Corolla with about 115K miles.  Recently the check engine light came on.  Since I've had this problem in the past and was able to fix it, I figured I needed to clean the mass airflow sensor, which I did with a spray electronics cleaner, and I also changed the air filter.  The check engine light came back on in 10 days.  I cleaned the MAF sensor again, but the light came back on in 1 day.  Throughout this process the engine seems to run fine.  Since a new sensor is pricey, is it advisable to replace the sensor with a used one?  Please offer suggestions.  Thank you.

ANSWER: Hello, what is the diagnostic code number you are getting ?

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QUESTION: Yes, thank you, I should have checked this first.  The code is P0446.  Definition: evaporative emission control system vent control circuit fault.  Probable cause: 1) open or short circuit condition; 2) poor electrical connection; 3) faulty CCV vent control solenoid.

This is a new one for me -- what do you suggest?  I'd appreciate as much detail as possible.

ANSWER: Thanks, that code is not related to the maf sensor, it is a large leak at the fuel recovery system (evap system) try replacing the gas cap, check the fuel filler neck for any signs of damage, clear the code and see if the light comes back, the 446 code is usually a code for the fuel tank not holding pressure.

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QUESTION: I replaced the gas cap with a new one, cleared the code, but got the check engine light again.  The problem may be the fuel filler neck -- about 3 inches down there appears to be some corrosion.  I can't explain how it got there, nor can I tell if it goes all the way through.  Is there a product I could apply to seal it?  Is replacing the fuel filler neck a job a do-it-yourself-er could reasonably do?  I appreciate your suggestions -- THANKS.

Answer
Replacing the fuel filler tube is not that difficult, you'll have to remove the wheel and the inner fender liner, may be a good idea to do that first anyway for inspection, you might also consider having the system tested for leaks using a smoke machine, this puts a tracer smoke into the evap system so the leak can be pinpointed, most auto repair/emission control stations should be able to do this