2002 Toyota 4Runner

Hello !

Did you check both Oxigen sensors?

Oxygen sensor 1 is often informally called the "primary" sensor, or "upstream" sensor. It's installed after the exhaust manifold, but before the catalytic converter. Oxygen sensor 2 is informally the "secondary", or "downstream" sensor. It's installed just after the catalytic converter.

With the extreme sensitivity of current North American emissions controls, and the extremely low emissions limits that have been imposed, certain errors like the P0420, which is supposed to mean that the catalytic converter is not working as well as it should. Unfortunately, it just means the downstream O2 sensor thinks too much oxygen is sneaking past the cat and tattles to the ECM.

A catalytic converter is an oxygen storage device. The only way it can function is if it can take up and release oxygen in the quantities required to convert engine emission gases to water and carbon dioxide. OBD-II specifications require that the catalytic converter be regularly tested by the ECM. Test failures tell the computer that the cat has lost some of its oxygen storage capability, which is what's meant by "below threshold".
Your car's computer is allowed to adjust the fuel/air mixture within a very narrow range in order to help keep the P0420 error from happening. If the needed adjustment exceeds that range, the error code will be set.

The next step its to check both O2 sensors and be sure that they are in between the specifications.

Good Luck!