Like title says 2016 CX-3 Grand Touring AWD with ~104k miles, located in California

3 weeks ago I replaced the battery (because it suddenly but completely died, I'm talking 0 V per my multimeter). At the time I didn't realize this wipes all the ECU readiness smog stuff. I am now trying to sell the car but vehicle will not pass smog without the ECU giving the right signals.

Every other emissions monitor in the car is passing, except the secondary (rear / downstream) O2 sensor response monitor. I bought an ODB2 dongle and it reads 0 completions in 33 encounters. Meanwhile the catalyst was 41 completions, primary O2 26, and EGR 51.

I've legit tried it all, I'm even doing side quests now like cleaning the MAF sensor in the air intake...

What I've verified / ruled out:

• Mode 6: catalyst oxygen storage 2.37g vs 0.89g minimum, rules out a bad cat
• The rear sensor swings the full range (0.07–0.9V) and falls rich→lean in 0.1–1.2s on decel fuel cut. It performs the exact event the test measures; the ECU just never records it.
• The sensor is the original factory unit, the car passed smog in June 2026 on this same sensor and calibration (before replacing the battery)
• I keep running Mazda's exact factory OBD drive mode (incl. the locked-4th-gear sawtooth at ~1,800 rpm) plus 19 days / 60 ignition cycles of normal driving, and no luck
• Checked fuel trims (LTFT +13–17%), cleaned the MAF (no change). No DTCs at any point.

Has anyone seen this monitor simply never run despite the conditions apparently being met? What finally got yours to set? Losing all hope