How can i trick my computer?

i replaced my fan a while ago becuase the OE one was too noisy.
ever since then, theres been a error code during start up -- cpu fan failure even though theres a working fan with one speed. there are 4 wires coming out of the mb but the fan has 2 wires pos and ground...Is there a way to "trick or fool" my computer into seeing the "right" voltage?

the computer is a dell dimension 5150c

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Check inside BIOS settings. There is usually an option for you to disable the checking of fan status, among other things such as temperature sensors.

That said, I think it's better if you figure out the root cause of the problem, rather than hiding the error message. It's likely because the fan you replaced is just a normal DC fan (2 wires) and does not send its status to the motherboard.