PO135 and PO141 Fault Codes on 2000 Ford Focus - O2 Sensor Replacement Help

I'm experiencing both PO135 and PO141 diagnostic trouble codes (DTCs) on my 2000 Ford Focus, even after replacing both oxygen (O2) sensors. I'm relatively new to using an OBDII scanner, so I followed the common advice to replace both sensors—especially since the engine has reached approximately 120,000 miles. However, I'm now wondering if my troubleshooting approach was incomplete. Should I have first checked for a blown fuse or proper voltage supply to the oxygen sensors? Additionally, could cleaning the Mass Air Flow (MAF) sensor help resolve these issues? I recently installed Bosch O2 sensors, which have slightly different physical designs and shorter wiring harnesses—but they connected properly. Any guidance on next steps would be greatly appreciated. Update: I located the heated oxygen sensor fuse and tested it—there was 12V DC with ignition on, so the fuse appears to be functioning correctly.