2008 Mazda 2 ~ 1.5 sport. Code P0031

Mazda 2 2008 1.5 sport. Code P0031

After removing the air/fuel sensor then having to cut wires to re-install it ( long story), I keep getting P0031 error code.

There are two black wires, a blue and a white. After cutting wires to screw the sensor back in and re-joining wires I keep getting P0031 code. I`m convinced I`ve joined the two black wires the wrong way round but there is no obvious difference between them. Is it possible I`ve blown a fuse/relay? I`ve tried reversing black wires around but still get P0031 code. Long story as to why I removed the sensor but it was working before I removed it. My user manual is useless and doesnt locate the actual fuse/relay. Not even sure if it even has one. Anyone with an idea?