I have a 2006 Toyota Matrix 4WD with 94000 miles.  About 9 months ago, the check engine light came on.  I took it to auto zone to get the code read, and it came back with P0420.  I took the car to my mechanic (who replaced the exhausts system from the manifold back, including cat,  about 2 years ago), and they tried cleaning the mass air flow sensor, and cleared the code.  The check engine light stayed off for about 3 weeks, and then came back on.  I let it go for a while because the car was running fine and not due for inspection.  I did try cleaning the mass air flow sensor myself, I replaced the air filter and bought an OBDII scan tool to read the codes myself.  I would clear the code after each attempt and a few days later the light would be back on.  I tried Scotty's Lacquer thinner in the gas trick to see if it would clean it out. No luck. I took it in to the mechanic with the light on and he tried replacing the MAF and cleared the code.  Currently the code is in the "pending" area but the light is off for now. This car has a second catalytic converter in the exhaust manifold, which the mechanic thinks should be the next step.  Unfortunately, they don't seem to be available.  Toyota lists the part as obsolete. I am not sure where to go from here.  This has been a great car.  It gets great gas mileage and is good in the snow with the 4wd.  I hate to see it become unusable just because I cannot get a cat for it.  Suggestions?

2006 Toyota Matrix 4WD 94,000 miles