how do I relock bootloader? topic

I got an update on my Xperia M2(d2305) to 4.4.4. I rooted my phone when it was 4.4.2. When I try to update it, it shows an error where it says (Sony update engine failure error 2003). I unroot my phones using the factory data reset but still with the same error. When I checked the bootloader it still say YES. .how do i lock back?