R7 240 not working with PCIE 2.0 motherboard.

Apr 17, 2018
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So I have an older computer that I want to turn into a gaming pc for my brother. It has a Core i5 750, either 8 or 16gb of DDR3 ram, and an AMD 4300/4500 HD series graphics cards. I have a 300W 80+ Bronze psu. I tried replacing the AMD 4300/4500 HD Series graphics card with the R7 240 and it gave the 5A error code. I looked it up online and it means there is something wrong with the graphics card. I know it is not dead because I gave it to one of my friends to put in his older system and it works perfectly. It may be because the graphics card is PCIE 3.0 and the motherboard is PCIE 2.0 but I've heard it is backwards compatible so...