Dell fans have an extra component mounted on the fans.  The BIOS checks (detects) for that and if not there you will get the fan failure notice.  Non Dell fans will work OK, just the notice at power on and having to manually press an F Key to continue the boot.      
Hi everyone, I recently purchased a Dell Optiplex 990 that came with an i7-2600. I set to work on upgrading basically everything else. I doubled the RAM to 16gb, I upgraded the power supply to a 500w, I installed a GTX 1060 6gb. Those all work perfectly so far. The new components that are causing me issues are the new CPU cooler and chassis fan I installed. I got a Noctua NF-B9 redux-1600 PWM for the chassis and a Noctua NH-L9x65 for the CPU cooler. I had to also get some 4-pin to 5-pin adapters for these components which may be the problem. Anyway, booting it up, I get a message at startup saying there are errors with those two fans. However, they're working fine, running SUPER quiet, and the CPU cores are never going above 55 degrees Celsius. My question is, should I worry about these fans? They appear to be doing their job. Are the errors simply from the motherboard not recognizing the adapters or something like that? Should I disable these errors and just regularly monitor the fans and temperatures if that's the case? And if so, how do I do that? Thanks!
Upgraded Optiplex 990, issue with CPU cooler and fan