It's the post cat sensor that trips the P0420. If you can monitor the post cat O2 sensor voltage when warmed up and at a steady speed. It should be held steady above .550v or so, usually near .700v. the volts will drop when you let up on the throttle for any length of time.

If it is rapidly cycling ( like the pre cat O2 does) the cat is bad or if at steady .450v or lower, the sensor is bad.

 

how do i monitor the post cat sensor? what tools should i need to have?
I have got OBD2 scanner if its helping

 

The O2 sensor voltages are a standard OBDII protocol so any OBD data reader should be able to see them.

 

today, i went to a shop which perform "4 gas emission test" and the owner of the shop told me that the results are absolutely fine and to not buy a new catalytic converter.

Why not ask what they thought the problem was?

 

post O2 sensor is after the cat. The rear sensor

 

I had a car that would occasionally throw a P0420 code. The problem was a small leak in the exhaust near the second or post O2 sensor. This leak can even be slightly down stream of the sensor. If your car has a flex joint in the exhaust, it may have a small crack that opens up when the engine twists under load. A good inspection of the exhaust system may be in order.

 

You need to see what it is doing while driving down the road at a steady throttle. If it stays near .65v and doesn't waver much the cat is good.

I think you have to focus on these 2 remarks.

P0420 I had this error on my Toyota Avensis mk 2003 , that car was really using oil and that poluted the cat so efficiency dropped and that fired a P0420 code.
If you remove the cat and soak the inside in citric acid you are sure the cat will work fine for a while.
For me it helped for 6k mile before the next P0420 code appeared.

 

Make sure you have a clean air filter. If you don't, crap can be pushed past the air filter and into the engine and contaminate the CC and sensors.

 

Maybe if a rodent built a nest in the air box. Still not likely to ruin the cat or O2 sensors.

 

Faulty diagnosis. Needs to be completely warmed up. Rear should not move much compared to front.
A scan tool is only as good as the person using it. You need to learn what these numbers mean.

 

Faulty diagnosis. Needs to be completely warmed up. Rear should not move much compared to front.
A scan tool is only as good as the person using it. You need to learn what these numbers mean.

so maybe you can help and tell me what those numbers mean so i can do a proper test next time

 

0.455v and no change is a dead sensor. That is the default ECU value that is used normally until the post cat sensor warms up and starts working, usually in under a couple miles.

Be sure the O2 fuses are good!

 

These are cold start values.

 

scannerdanner.com forum is a good place to post the symptoms of your car and the numbers provided by the scanner. There's a good group of guys over there and I hate to see components replaced without being sure they are faulty.

 

It is the frequency and amplitude with which the second O2 sensor varies. If it exceeds limits it throws a code. The scanner might pick up something called catalytic efficiency. If I remember correctly, it's allowed to drop to %96.

 

video upload will help you ?

 

thanks

 

Hard to read but looks steady around .650V? But is this at idle or driving down the road?

 

idle on about 850 rpm

 

FWIW, the test the ECU does happens after the engine is good and warm. Then shut off and falls below 150F but not to dead cold. Then as soon as it hits 150F again it does the test and will trigger the light if it fails.. You can watch it on the O2 voltages.

 

This is the Front/Upper Sensor values:

 

This is the Front/Upper Sensor values:
View attachment 464301

Guessing from the pre cat O2 values you should be seeing something around a steady .550-.600v or there abouts on the post cat O2. When warmed up and driving down the highway.

 

Your post cat (#2) sensor shouldn't flux like the pre cat sensor and it shouldn't react to throttle in just 2 seconds. Seeking that tells me the cat is not storing oxygen. You need a new cat.

 

its reacting exactly in the same moment that im throttling. i think you didnt understand what im saying.
i'll upload the video i took the values from later on maybe that will help me to express myself better

 

Je me permets de poser cette question, as-tu effacé le le code d'erreur après les réparations? Sur certains véhicules ceux-ci restent allumer même si le problème est réparé. Sinon, je te conseille de changer le capteur inférieur, de vérifier attentivement qu'il n'est pas de petite fuite d'échappement. Ensuite certain teste plus pousser effectuer pas des professionnelles pourront plus t'aider. il vont vérifier l'efficacité de ton map/maf, de tes injecteurs et dans des cas plus extrêmes le Timing entre tes valves et tes pistons.