Originally Posted by boston_kevin

And the Verdict is:

Oxygen sensor in back of one of the cats. Tech showed me the code on their computer when he plugged it into my 4Runner.

This is how the tech explained (and showed me) how this works. 4Runner has 4 Oxygen sensors (1 on either side of each Cat). Engine side sensor monitors "dirty air", muffler side sensor monitors "clean air". If either sensor (or either side) fails, it throws the CEL. Makes sense to this point, right? On 2004+ models (and if you had the Sulfur TSB), if the CEL is lit, it also DISABLES the VSC/TRAC systems.



So I asked the question, why disable these systems if the CEL is lit? He said he did not have a direct answer but basically Toyota thought if the CEL light was on, any system that directly put a load on the engine that wasn't necessary should be disabled (which also means that the DAC might also not work) to prevent further damage to the engine. Since the VSC/TRAC works in conjunction with the brakes which can add a load to the engine, this system is disabled.

I asked one more question (for all you modification happy people who don't like VSC). Can I purposefully disable the VSC? No (officially). (unofficially you could throw a switch on the wire from one of the sensors to spoof the computer to think the sensor failed - since this would force a CEL condition, VSC would be disabled).