Hey all,

I drive a 2005 Mazda 3 S 2.3 liter with a 5-speed manual transmission and 139k miles on it.

I have a code P0421 and just need to figure out if I might have an issue that is sensor related rather than a bad catalytic converter. My downstream O2 sensor was oscillating like my upstream sensor, so I replaced it with a Denso replacement. The graph still reads the same, so I thought to just unplug the sensor while the car is running and see if anything changes.

Per my research online, the downstream sensor's ODB2 reading should just flatline to the lowest y-axis value (zero, 1.28 volts, etc). In my case, the reading doesn't change at all. I'm curious to know if this means there isn't an issue with the O2 sensor but instead an issue up the wiring that goes to the ECU. I don't really know how to check this or what to ask/do next.

For background, I got my clutch changed in early January 2025 and received the car with a battery light and CEL P0133 and PO421 (replacing the upstream sensor fixed P0133). After returning the car for them to check, they said there was some wire that needed to be reseated in the ECU which fixed the battery light issue but did not fix the CEL's. They maintained that the catalytic converter is bad, but the only diagnosis they were willing to do was just look at the downstream O2 sensor's live stream. It helps to note that the wiring harness protecting the sensor wires to the ECU travels directly over the clutch housing.

I decided to cut my losses with that shop and just do my research on it instead. Plus I don't need to pass smog until July 2026, so I have alot of time to test things before spending big bucks on a cat replacement. The car runs a beautifully as ever, anyway. There's no foul exhaust smell, gets perfect gas mileage, great throttle response, engine braking, etc.

So as a result of the above, I'm thinking the issue at hand is a wiring issue that's causing the ECU to ignore the downstream sensor and just do "something else". I'm saying this without truly understanding how the ECU works in my car, which is why I would like your input in case any of my logic is on the wrong path.

Thanks all

What should OBD2 live data show when downstream O2 sensor is unplugged?