I have the P0420 code on my 1998 Honda Civic DX (catalyst system efficiency below threshold bank 1). I want to be able to properly diagnose the problem myself without just throwing new parts at it. A new catalyst is fairly expensive. I used a scan tool to log some data from the O2 sensors as I drove around. My question is how do I interpret this data and can I determined that it is the catalyst and not the O2 sensors that have failed?
First I let the engine idle for 15 minutes to reach operating temperature. This is the graph of the O2 sensors at idle:
From my research I can say that the upstream O2 sensor (red) look okay. It is fluctuating between lean and rich conditions. The downstream sensor (green) is stuck at around 0.9V. I know that the sensor is supposed to be a flat line around 0.5V so this is a problem. It is not clear to me whether this is due to a failed catalyst of a failed sensor.
Here is a graph of normal driving:
Here is a graph of hard acceleration:
Finally, here is a graph of coasting down a hill. You can see where the computer shuts off the fuel injectors.
These graphs make me think that the downstream O2 sensor (green) is not faulty. Especially the graph from coasting. When the computer shuts off the fuel injectors, the intake becomes extremely lean and both sensors respond immediately. If the downstream sensor was faulty, it should stay stuck at a high voltage. Is this interpretation correct? Can I conclude that I need to replace the catalyst?