O2 sensors have to be hot to work, so many of them have an electric heating coil wrapped around them which heats them up so they start working faster after starting the engine, which reduces the car’s emissions. The O2 sensor still works without the heater working b/c it gets hot just by being in the exhaust stream, but it takes a little longer to start working is all. So your Mazda probably runs like it always did right? If so, this isn’t an emergency.
B/c it is emissions related, even tho it only affects emissions level for the first few minutes after starting the engine, the EPA probably requires the heating coil work. The engine computer outputs the voltage to power the coil, and monitors the voltage to make sure it is working. Somethings wrong with all that. From what I understand you’ve verified the heating coil voltage at o2 sensor, but it isn’t getting back to the engine computer. The things that could cause that,
- corroded or broken connecter at the O2 sensor
- corroded or broken connector at the engine computer
- broken or shorted wire between the O2 sensor and the engine computer
So it is like if your friend starts from their home but never arrives at yours. What would you do? Go to their home and trace along the path on your map they’d take, step by step, to find out where they went missing. That’s what you need to do for your O2 sensor problem too. Map is the key word, as this could be near impossible to do without one. Secure a wiring schematic for your car, and have a go at it. Or if you aren’t familiar with how to do that kind of thing, ask your local auto electric shop to have a go at it, using your wiring schematic. Best of luck.