@sachincng could be an issue with the keyboard IC on the motherboard or it could be a failed keyboard. Most commonly it is the keyboard that has failed and needs replacing.
Use the [document|39853] and on page 46 you'll see how to replace the keyboard.
While you work on your laptop, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy!