2003 Highlander CE light, scan codes P0420 and P0430. DO the appearance of both 0420 and 0430 indicate that problem is most likely the Catalytic converter? Or might it still be a problem with either or both sensors? Or something entirely different? Also I see a Catalytic converter on the exhaust manifold (almost dead center in the engine compartment) is there another one in the exhaust/muffler line path under the car too?

SOURCE: check engine light

that code is saying that your catilitic converter is junk, but could be caused by some other problem also gas leak on exhaust system a/f sensor bank 1,2 sensor 2. heated oxygen sensor bank1 sensor 2, or the three way catilitic converter. these are the listed reasons for that code being thrown, to tell you the truth its not really that big of a deal ya know, i mean its just emissions, either the cat is bad or the oxygen sensor is bad in my opinion will it hurt the car? no, will that little light annoy the **** out of you? YES ; ) because the sensor is behind the cat it will not have any imput into your computer to change fuel maps or anything, the post sensors are only for reading if the cat is good or not, i actually cut mine off and the cat, the check engine light might always be on, but if my car messes up for some other reason then i will feel or smell it and dont really need the dummy light yea know? anyways its your choice to live with it or to take it to the shop and have your sensor tested/cat looked at, either way it wont hurt your car, if your front o2's start going bad you have to change those, but not the post cat one. dont let those shop guys bully ya into spending to much tell me what they say cause there always after an unsuspecting person's wallet, thats why i opened my own shop.

if this helped throw some blue/fix ya rating.
take care have a good day
EriK
Ez Automotive/U.S. Army

SOURCE: 2003 toyota camry

P0420 is a cat code. Sometimes, you can replace the upstream & downstream air/fuel & O2 sensors, run some good fuel injection cleaner thru it, and that might fix the problem. If not, you may need to replace the cat. Note that the cat is covered under warranty for 8 yrs/80,000 miles. Toyota will replace it if it has gone bad, for free.

SOURCE: Help with toyota corolla 2003 code P0420

CODE PO420 CATALTYLIC CONVERTER SYSTEM FAULT.ITS SAYING FAULTY CATALTYLIC CONVERTER.IF IT BEEN A WHILE IF YOUR CAR HAS A WHOLE LOT OF MILES ON IT.YOUR SPARK PLUG CHANGE IS DUE AT 120000 MILES. DUE A LITTLE TUNE UP CHANGE AIR FILTER AND FUEL FILTER.SEE IF YOU HAVE ANY ENGINE FAULTS BAD PLUGS DIRTY INJECTORS ANY THING UP SET FUEL SYSTEM CAUSE CAR TO RUN RICH.EXCESSIVE UNBURN FUEL WILL GATHER IN CATALYTIC CONVERTER CAUSE IT TO BURN OUT.SHORTING LIFE OF THE CATALYTIC CONVERTER.

SOURCE: P0420 and P0430

The AF sensors are uupstream and the O2 down stream. The car' s computer tests them and it is saying your cats are bad. If you have a scan tool you could look at the data and see what the sensors are seeing. The Af sensors are the ones that control the fuel ratio in the car while the O2's just watch. AF's would be the one to change but there is no garuatee that the codes won't come back.

SOURCE: vsc and check engine light is on

DTC P0420: Catalyst System Efficiency Below Threashold (Bank 1)

The ECM uses sensors mounted before and after the three-way catalyst (TWC) to monitor its efficiency. The first sensor, and Air Fuel ratio (A/F) sensor, sends pre-catalyst A/F ratio information to the ECM. The second sensor, a heated oxygen sensor (O2S), sends post-catalyst information to the ECM. The ECM compares these two signals to judge the efficiency of the catalyst and the catalyst's ability to store oxygen. During normal operation, the TWC stores and releases oxygen as needed. The capacity to store oxygen results in a low variation in the post-TWC exhaust stream as shown below.
If the catalyst is functioning normally, the waveform of the heated oxygen sensor slowly switches between RICH and LEAN. If the catalyst is deteriorated, the waveform will alternate frequently between RICH and LEAN. As the catalyst efficiency degrades, its ability to store oxygen is reduced and the catalyst output becomes more variable. When running the monitor, the ECM compares sensor 1 signals (A/F sensor) over a specific amount of time to determine catalyst efficiency. The ECM begins by calculating the signal length for both sensors (for the rear oxygen sensor, the ECM uses the output voltage signal length). If the oxygen sensor output voltage signal length is greater than the threshold (threshold is calculated based on the A/F sensor signal length), the ECM concludes that the catalyst is malfunctioning. The ECM will turn on the MIL and a DTC will be set.
Hope this helps :)