System Fan Failure?

Many computers use a tachometer of sorts to measure the speed at which certain fans are spinning. If as dingo07 mentioned, the fan attached to the connector Dell has identified as "system fan," failed to spin during a check of the tachometer, it makes sense the warning was thrown. Fans that have become clogged or have bearings that have either failed or are in the process of failing, tend to have fluctuations in their rotational speed. It's likely the fan started spinning fast enough to prevent the error, after you power cycled the machine.

It's best to have the computer checked out internally at this point, either professionally, or if you're up to it, you can most likely do this yourself. Just be aware, fan bearings are not meant for high speeds so, if you use compressed air or a vacuum cleaner, prevent the fan blades from spinning to avoid bearing damage. Replace any fans that are failing and clean the rest, especially clogged heat sinks.

That's great, but be warned - there's moisture in the air coming from a compressor, especially one in a dusty workshop! You could possibly cause a short if the machine is on, or enough moisture gets on the boards. A can of air is much better and a vacuum helps too

No, the fan has no bearing on whether the computer turns on. The computer has most likely overheated and shut down. At this point it's damage control - the processor is probably dead. Unplugging the power for a couple minutes might reset the motherboard. And no, the firmware doesn't operate like that. When you press the power button, it either comes on if all's good or it doesn't.

Did you ever check to see if the fan on the processor spins normally and doesn't get stuck, or his hard to spin?

It sounds very much like the "system fan" in your machine is the CPU cooling fan, as others have suggested. And it sounds like its bearings have worn out so the fan sometimes works (maybe at slower speed than intended) and sometimes just stalls. A fan in this condition may fail to start up reliably at boot time but then start a bit later. Or, it may actually stall after running for a while when its bearings heat up. In either case, you MUST replace this fan!

Now, your machine MAY work OK once the fan is replaced, maybe not. The hope is based on this: there are probably two protection systems for your CPU chip against overheating. The first is ALWAYS there in every system. The BIOS monitors the actual temperature inside the CPU from a sensor built into the chip; if it goes high, the BOIS either slows down the whole system to let the chip cool or, if that is not enough, it will shut down the system. Of course this means that, after it has been off for a while and the CPU temp is cooled off, it will re-start normally. BUT the original cause of overheating is still there, so it's very UNWISE to do this now.

Many mobos also have a second protection, and maybe yours has this. It monitors the speed signal being sent from the CPU cooling fan to the mobo. If that signal fails it indicates that the fan has stopped. (Note that this could happen if the fan really does fail, OR simply if the fan's speed signal fails to get to the mobo because there is a bad connection where the fan plugs into the mobo port.) If the mobo senses that the CPU fan has failed, it sends out a warning and usually completely shuts down the system within a short time. It does not even wait for the CPU temperature to rise. This is a pre-emptive way to prevent CPU overheating. If this is what has happened, the next time you try to turn on the machine it could go two ways. If the fan has failed and it does not start up (or even if the signal merely does not get through), the system will warn you again about the system fan failure and will not start. But if the fan does start up the system will work. THEN you have the worry - when will it fail again?

Bottom line: when you get dire warnings, pay attention! A failed system fan (CPU cooler) MUST be fixed before you do anything else. Ignoring that problem leaves you very vulnerable to CPU overheating and permanent damage, even though the system tries to protect itself against this. In your case, do not start up the system again until you replace the fan. Hopefully there was no permanent damage, and your system will work again when you have good CPU cooling.

So bottom line, if the faulty fan is the reason your computer fails to POST correctly, replacing it may well solve you problem. You need to investigate this yourself, or have it looked into by a professional. We don't have enough details yet to tell you with certainty. For what it's worth, a fan in such bad condition can usually be spun with a fingertip while the system is powered off, and you will see that the fan will either not spin at all, or stop spinning once you stop applying force.

As Paperdoc mentioned, there is more than one possible reason your system may be failing to POST. Also be aware, whether your system implements one method, both, or neither, will depend on the make and model of computer parts in your system.

When a power supply detects seriously abnormal voltage levels, it shuts the system down. If it were far enough out of spec to cause your fan to stop spinning, your computer would be entirely unable to display an error message, much less run at all.

It may very well be that your power adapter as you are calling it, is faulty, but that doesn't explain the "system fan" warning.

On the other hand, power supplies in computers are usually actively cooled by a fan. If the fan that was failing happened to be the power supply cooling fan, and you continued to run the system, your power supply could have failed due to overheating.

Well, most standard desktop style computers have a power supply fan. Looking up your particular make and model of computer, it appears you have an all-in-one style, and your power is supplied from an external adapter, not an internal power supply as would normally be the case. The construction of your particular computer is much more like a laptop, and it appears, there is a single fan to cool the entire system.

Here is the service manual for your computer, which should help you in locating and replacing your system fan: SERVICE MANUAL

Hurray for success! Glad your machine is working.