[kathyricks]

The P0420 Catalyst System Efficiency Below Threshold check engine light seems o be commonly showing up nowadays on the late 90's and early '00 Toyotas at around 90,000- 150,000 mile mark. Sometimes replacing the oxygen sensor in front of the cat makes the light go away, other times it doesn't and the converter has to be replaced.

Prevention is the strategy that will likely avoid future wallet shocking P0420 lights; i.e. you need to do what you can to reduce the concentration of unburned hydrocarbons the engine generates and which enters the cat. The way to do that is to monitor your fuel economy and start replacing or cleaning parts as needed to restore fuel economy; e.g. changing the air filter, spark plugs and cleaning the throttle plate of deposits. And avoid cat destroying practices like frequent full throttle acceleration and adding engine mods like aftermarket air intake and exhaust system mods. And never use aftermarket parts of any kind (e.g. spark plugs)