"Some say" that a threaded sleeve which moves the tip of the REAR O2 Sensor up and away from the inside of the tailpipe will do the trick. That rear sensor is used as the "sniffer" and OBDII "plug in to the socket under the dash" emissions checks use IT for readings of the content of the exhaust. As one guy says, "It rats you out to (your) ECU" and lights the CEL. You could, I am sure, play around with a resistor substitution box. ON the early OBDII systems, the front O2 OR "Air Fuel" (in California) systems sensor (or sensors on V engines) only provide feedback to the ECU for mixture adjustment. There is an extra Air Fuel sensor on 2001 and later Toyotas but I am not sure what it measures, probably FRONT converter function. Air Fuel sensors are a lot more expensive than plain O2 Sensors, and are NOT interchangable. I am "aware" of a Toyota product with OBDII which has the front "precat" mounted IN the exhaust manifold and that cat (called a "precat) is almost totally blown out. Nevertheless the car is stil passing OBDI inspections with 160K on the clock. To change tha "precat at stealer prices is upwards of $800 bucks, not including "collateral damage" as the service droids screw up your exhaust manifold studs on YOUR dime. The key is to have the plugs in good shape (less for the precat to do) and a REAR cat in good shape. There is an OBDII code for "rear cat operation below minimum" (PO 430) before you go nuts changing cats, make sure the engine is well tuned up. If you live in a state like Florida, where there is no smog test requirement, they don't GIVE a bleep about what ANY of the sensors do. You can rip them all out, and take the bulb out of the cel, but you don't want the engine into "limp mode" so keeping the front sensor is required for proper FI operation ONLY.
