found this about P 0420
 

If anyone is interested on it. still slightly confused about it.

--P0420 Catalyst efficiency below threshold. My favorite, Here is why the code is set. Starting in 1996 every vehicle for sale in the U.S. had to have a way of monitoring Catalyst function. The job of the Catalytic Converter is to take Carbon Monoxide (a by product of internal combustion) and convert some of it back to Oxygen, this is done with three precious metals, temperature & fuel mixture. There is an Oxygen Sensor, or O2 sensor mounted in front of the Catalyst and one after. The ECM (Engine Control Module) uses data from the O2 sensors to know if the Catalyst is doing its job of creating Oxygen. Now the ECM does not constantly monitor the Catalyst system, this starts to get a bit complicated but depending on how the vehicle is being driven it is not always possible for the Catalyst to function properly as such in the ECM’s programming it will only monitor the Catalyst function after certain events have been met in a drive cycle. Once the ECM is looking at Catalyst it uses the voltage values created by the front and rear O2 sensors to determine if the Catalyst is working with in the programmed threshold. there are a couple of ways to properly test a Catalyst, but the most accurate way is to do a Catalytic Converter efficiency test. This test involves running the vehicle until it is up to temperature, then install a 4 or 5 gas analyzer probe into the tail pipe of the exhaust, then shutting the vehicle off, disabling spark and fuel (to keep the vehicle from starting), injecting propane into the intake manifold, and cranking the vehicle over and obtaining the readings from the gas analyzer. The reason for the test procedure is to remove the ECM, Mixture and O2 sensors from the loop, to purely test the Catalyst’s ability to function and it is a pass or fail test with a few Grey areas. It is not possible to simulate this test with a generic scan tool in the Auto parts store parking lot. A failed Catalyst is just one of many possible causes of this code and at the time of writing this article the list price for a one piece Catalytic Converter (found in most 1999 to 2004 models) from Subaru was $974.00 not including labor or gaskets. If the Catalyst is not the cause which is very possible and the culprit is a O2 sensor there are other possible ramifications of not repairing the vehicle properly. If the Catalyst has failed, reasons as to why need to be investigated to avoid a quick repeat of the failure. The number one cause we see at the shop is a failed head gasket allowing coolant into the exhaust and degrading the Catalytic Converter and or 02 sensors. But we have also seen contaminated gas, slow O2 sensors, and Mixture problems cause this as well. You have to remember the Catalyst function is measured by the ECM as seen through the “eyes” of the O2 sensors. If you live in Washington State and fail an Emissions test as a result of a check engine light and a PO420 code set, you can request they perform a tail pipe test and if it passes so should your car. Why is this? There is chatter that there is a disconnect in some cases between oxygen content in the exhaust and cars that will still run below the legal “measured” tail pipe Emissions standards for that year.