CODE P042 Tollota Corolla CE 2003

Hello, I have a problem with this code P0420 (catalytic converter efficiency below threshold, referring to bank 1). It's a Toyota Corolla CE with a 1ZZ VVT-i 16-valve engine.
Which sensor is causing this fault, where is it located, and what should I do? Thank you in advance for your help.

If it refers to the catalytic converter, it should be a problem with the oxygen sensor on bank 1, in my opinion.

Hi, this description will help you understand the problem. First of all, you should make sure you don't have any leaks in the exhaust system.


DTC:P0420 Catalytic converter system efficiency below threshold (bank 1)
DESCRIPTION
This vehicle has two heated oxygen sensors. One is located upstream of the three-way catalytic converter (TWC), and the other is located downstream. The signals emitted by the sensors are converted into waveforms within the ECM. The upstream heated oxygen sensor allows the ECM to monitor the air/fuel ratio, and its signal prompts the ECM to control the air/fuel ratio feedback. As a result, the air/fuel ratio is balanced, and the waveform of the upstream heated oxygen sensor oscillates regularly between rich and lean. To determine whether the TWC's performance has deteriorated, the ECM compares the waveforms of the upstream and downstream heated oxygen sensors. While the TWC is functioning normally, the waveform of the downstream heated oxygen sensor fluctuates between rich and lean more slowly than the waveform of the upstream heated oxygen sensor. When the heated oxygen back probe waveform fluctuates frequently, it indicates that the TWC performance has deteriorated.