XFX Radeon HD 6850
Have you tried to open a XFX Warranty ticket to see if they will replace the fan? Some manufacturer's have a lifetime Limited Warranty but you need to check with XFX Support to find out. If it is not under Warranty see if XFX will sell you a replacement.
Otherwise you always purchase a 3rd party fan that is compatible with your HD6850 Cowl or covering.
Here are some 3rd party fan replacement:
75mm Sapphire XFX HD4860 HD6850 HD6870 Fan Replacement 39mm 2Pin FD8015U12S R89a 712424174549 | eBay
https://www.amazon.com/Graphics-Card-Fans/b?ie=UTF8&node=3015421011
Note: you need to find out the exact Fan size your HD6850 uses before buying any replacement.
By the way, how do you know your GPU Fan is defective and not the GPU card itself?
My GPU is working all fine, as i am still using it, just that i cannot
perform heavy tasks, coz the temperature spikes.
So you are now saying your GPU Fan DOES WORK but not running fast enough to keep your GPU Card cool under heavy loads?
Can you post information of Fan speed while under heavy loads and during idle while running your GPU?
Nope... I'm saying my GPU is working but the fan is not rotating at all...
On normal working condition temperature is between 50°c to 70°c
If I play a movie or try to play a game, the temperature starts spiking and
once it crosses 100°c system shuts down.
Okay, that clears that up. Thanks.
There are two solutions to your problem, in my opinion:
1) defective Fan and you need to replace or repair it
2) Defective GPU card (bad temperature sensor)
I would try to replace the defective fan first since it is cheaper than getting a new GPU card. But at the same time, your GPU card is considered to be legacy and not supported with AMD Driver updates except for the last two from 2015 & 2016. If you can afford to upgrade your GPU this would be a good time to do it. Just make sure your PSU is strong enough for the new GPU Card if you do decide to upgrade.
When you turn the fan by hand, do you feel any resistance or noise?
Make sure there are not obstructions that might prevent the fan from turning.
Nope. . . I don't feel any resistance.. Coz it doesn't work at all...
Plus on more thing, If I let you know my system configuration will you be
able to tell me how to upgrade it... With the least budget...
I can try.
There is a website about the minimum PSU needed to run either AMD or Nvidia GPU cards which is very helpful. PSU REQUIREMENTS - RealHardTechX
Your PSU is the one of the biggest factors in determining what GPU card you can install. The Processor is another factor and as long as your Motherboard has a PCIe x16 x8 slot or higher to install the new GPU card. Another factor is the size of your computer case. Mid-Tower or Full-Tower will accommodate all large size GPU Cards.
black_zion is probably the best person that can suggest the best GPU card. He is more knowledgeable about the type of processor you have and the GPU card that can work best with it.
I can show you which newer GPU card will work with the current PSU you have installed in your computer. If you are not sure of the hardware or software installed.
Download a small free program called SPECCY which will tell you everything you need to now about your computer hardware and software. You can upload a image of SPECCY if you want.
i would recoment an AIO cooler my Hd6780 fan got noisy so disconected it put an Zalman rezernater 3 max mine is daily at 1004 gpu and 1150 1250mv try 1231mv sits at max of 63c 44c idle if overclocked otherwise 33c
good luck fellow hd6800 runner