I just purchased a 2006 r53 checkmate edition. The previous owner told me he coded out the stage 1 cooling fan. does anyone have any idea how I can reactivate it?

 

There is no way to "code it out" most likely the resistor went bad and they simply bypassed it, and you'll have to just replace it and reconnect the proper wires.

The way the low speed fan works is simply a resistor in the system, it's not an actual 2 stage fan, I don't remember the temp ranges, but basically at lower (hot) temps and when the A/C is on, the power is sent through the resistor, causing a restriction in current and therefore a slower fan speed, above that temp point, the fan gets full current therefore full speed.

If you reinstall the resistor, just make sure that the wiring is still all there, if the previous owner got rid of it also, the resistor wiring is, and needs to be, a heavier gauge (I think 10ga) than the full speed wiring due to the extra heat generated by the resistor.
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You can buy a replacement low speed fan resistors on ebay, amazon, other parts stores. If you're not comfortable with soldering a new resistor or if the previous owner really butchered things, you may have to repalce the whole fan unit.