oui en général il faut se pencher en premier sur les deux sondes en amont du cata et a l'arrière et la sonde oxygène puis le catalyseur

mais c'est chiant car c'est un code général traitant en moyenne de 32 causes possible selon les marques ...

J'ai un PDF sur le sujet dans mes sauvegardes .. j'en avais bien chié avec mes sub mais j'avais réussi à lui tordre el cou malgré tout !!

https://www.mx5world.com/phorum/read.php?10,186790

P0420 DESCRIPTION : The OBD Code for Catalytic Converter efficiency Bank One below
threshold

A normal catalyst will convert the air mixture in the exhaust stream. When the catalyst is
no longer functioning correctly, the air mixture either doesn't change at all or is changed
at a reduced level. This results in the rear O2 sensor being too active.

The P0420 code is a catalyst efficiency code. The ECM monitors the front A/F sensor
activity and compares it to the rear O2 sensor. When the rear oxygen sensor rich to
lean ratio exceeds a set value compared to the front A/F sensor, and all the other
parameters have been met, the ECM will determine the converter is out of federally set
limits. When the ECM has determined the vehicle has failed the test in two consecutive
drive cycles, the check engine light will come on.

The P0420 code states that it is a Bank 1 code. In six cylinder and some PZEV
vehicles, there are two separate converters. The ECM takes a reading from both
converters and then averages them to determine if the total converter system output is
below threshold. After you drive the vehicle and save the file, you will need to look at
the readings from both sides to determine if one or both of the converters are
damaged.

Possible Causes of Defective Converters

The converter can be damaged because of a previous problem that has been
corrected or it can be degraded because of age. It could even be physically broken
from the impact of a previous accident.
Any time the vehicle has a misfire condition, lean or rich, it can overheat the converter
and cause damage. As an example, low fuel pressure will cause a lean misfire
condition.
The front A/F sensor could be defective causing the ECM to over or underestimate the
fuel mixture. This will cause the converter to be overloaded and in turn cause the O2
sensor to seem too active. This can eventually cause the converter to be damaged.

The engine could be running poorly because of bad plugs, wires, coil, clogged or stuck
injector causing too much fuel and oxygen to reach the converter. If this problem
happens long enough, it can cause the converter to be permanently damaged.

code effacé.
130 bornes sans aucun soucis.
Les sondes ont été montée par un garage ,donc je leurs fait confiance.

Les bougies sont d'aspect normal.

Bon je vais continuer a rouler comme ça pour le moment.
Si le code revient ,je repasserai par ce post

merci pour vos conseils

a+
Fred