Is there a "rear fan" in Optiplex 745 SFF? - General Hardware - Desktop - Dell Community

Is there a "rear fan" in Optiplex 745 SFF?


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Is there a "rear fan" in Optiplex 745 SFF?

  • very simply, on the back of the MB on a GX745 there is what looks like a fan plug. I have this GX745 SFF motherboard in a GX745 DT (Desktop Form Factor) case, and aside from a BIOS errors below, it works perfectly.

    The BIOS errors are;
    1. Alert! Rear Fan Failure
    2. Alert! Air temperature sensor not detected

    I would suspect that the BIOS is looking for a fan with a thermistor (temp sensor) plugged in, just there is no place for a fan to go in the desktop form factor.

    Is there a fan somewhere else on the SFF box aside from the front (obvious). (I'm not sure if there is a fan in the power supply, but I'd assume that there is) So, where would this extra fan be? Under the HDD assembly?

    Anyone out there have a GX745 SFF that's actually looked inside the box?
  • There is a fan underneath the hard drive.

    The temperature sensor is located near the front fan on each of the four chassis. The locations are as follows:

      Sensor Location Signal Cable Mini-tower Integrated into the front I/O panel Shared with the I/O ribbon cable Desktop Integrated into the front I/O panel Shared with the I/O ribbon cable Small Form Factor Snapped into the front grill of the chassis in front of the fan 2-wire signal cable connects to the I/O board (shown below) Ultra Small Form Factor Integrated into the system board No cable necessary

    The image to the below shows the air temperature sensor on the SFF chassis. The 2-wire cable runs across the bottom of the chassis and connects to the I/O panel.

    Sensor Location on the SFF Chassis
    Image of the air temperature sensor on the SFF chassis.
    Error Messages

    If the cable from the control panel to the system board is disconnected, the system displays an error on boot:

    Alert! Air temperature sensor not detected

    This message indicates the system did not detect the presence of the sensor or the cable is disconnected. Reseat the cable and reboot to see if the message still appears.

    If the temperature of the sensor rises above 42° C (107.6° F), the system logs an error (Air Temperature Out of Range) in the Event Log accessible through System Setup. The air entering the customer’s system has exceeded the 42° C standard, and the system must now compensate with increased fan activity and noise.

    A NOTE indicates important information that helps you make better use of your computer.NOTE:
    If the system reaches 42° C and logs an error in the Event Log, it still functions properly. The fans are just louder than normal.

    Message Edited by patrickacox on 06-18-2007 09:50 AM

    Message Edited by patrickacox on 06-18-2007 09:50 AM
  • OK - thanks for the great explanation and pictorial! I was able to fool the BIOS by plugging in a fan, but that of course didn't get rid of the temp sensor - looks like I'll need to find a temp sensor.

    One thing that seems odd - a temp sensor next to the air intake... You'd think that the temp sensor would be near the rear, and as it detected heat, would tell the BIOS to increase the fan speed.

    Thanks again!
  • i have optiplex745sff with intel core to duo processor e4400 2.0gege hz

    at the start up error msg appear

    1. Alert rare fan failure

    press F1 to continue or F2 to run setup utality


    plz any one help me to fix this problem

    my mailing id is

    <ADMIN NOTE:Email id removed per privacy policy>

  • The Rear Fan is under the hard drive in the sff.


    release tabs (2)


    hard drive


    hard drive bracket

    1. Connect the hard drive fan cable to the system board.

    2. Connect the power and data cables to the drive.

    Replacing Rear Hard Drive Fan

    1. Follow the procedures in Before You Begin. <ADMIN NOTE: Broken link has been removed from this post by Dell>

    2. Remove the computer cover (see Removing the Computer Cover). <ADMIN NOTE: Broken link has been removed from this post by Dell>

    3. Remove the hard drive (see Removing a Hard Drive). <ADMIN NOTE: Broken link has been removed from this post by Dell>

    4. Turn the hard drive upside down, so that the hard drive fan is visible in the bottom of the drive bracket.

    5. To remove the hard drive fan:

      1. Lift the release tab on the back panel of the fan.

      1. Rotate the fan in the opposite direction from that indicated by the arrow on the back panel of the fan.

      2. Lift to remove the fan and its back panel from the hard drive bracket.

    DELL Hard Drive Blower Fan Plus Hard Drive Bracket and

    Fan Caddy for OptiPlex 760 / 740 / 745 / 755 SFF & Dimension 5200c SFF

    Dell Part Numbers: NY290, TJ160, NH645, NJ793

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  • I too get this warning: 

    1. Alert rare fan failure

    press F1 to continue or F2 to run setup utility

    I moved the  HD and turned the computer on, and the fan is running fine. The error seems to intermittent, but I want to figure out what is causing it. What other troubleshooting steps should i take?

  • i have the same problem with the rear fan

    i have dell optiplex 745, i bought it today and it shows the msg

    which states rear fan failure

    i opened the case and found that there is no rear fan in my computer

    i dont think i will be able find the fan here and buy it .

    i just want the msg to be removed


  • Did you ever resolve the rear fan failure? How?

  • I have not resolved it. Waiting for someone smarter than myself to come to the rescue

  •   The error usually seems to occur when a dual core processor is added to a 745 SFF with no disk fan present.  Dell (the bios) seem to think the system will then have disk which needs a fan. The opposite is true as the dual core gives off less heat than a pentium D.  A fix which worked for me was to run the sense wire (yellow in my system and at the at the edge of the 5pin connector) to both front case fan connector and also to the matching pin on the fan connector for the disk fan.  The rear fan then looks to be present for the bios.  You need a spare dell 5 pin connector to do this, Easy to get by buying an old dell 5 pin fan on ebay.  You should check the ebay picture to be sure it is 5 pin fan.  

  • This worked for me. I used an old CD Rom cable, trimmed off the lock so it would slip onto the motherboard header, pulled the yellow pin on the front fan connector and reinserted with a resistor lead. Lined up the pin/wire from the header and slipped over the resistor lead.

    I did have an issue when all four wires were on the header, the power supply indicated a fault. I trimmed off the three extras and reconnected with just the one. 


    I now have a great mediacenter box (XBMC) that boots superfast and i dont have to have a keyboard to press F1!


  • I know this is old post , but I hope that someone will put the pic of the wiring , since picture means thousand words and I am really struggling with this error.

    I couldn't find anywhere else better explanation for this problem but still...


    I mean, I have no fan on the back of HDD ;front is  only one working good  but I am not sure what yellow wire is in question. 






  • HDD caddy has fan. not actual HDD


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