My screen knock feature don't work anymore

[Android Central Question]

Before I sent my LG G4 to LG repair center, my screen knock feature worked. On return, that feature is dead. LG won't even tell me what is wrong and won't fix it until I paid for diagnosis. I have tried everything in settings but to no avail. I am pissed off to wake up my phone by pushing the power button on the back especially if it is in a holder being used for navigation. This is ridiculous. LG Lg lg, you are getting on my **** list.....fix the problem. ANY IDEAS or SOLUTION?