How to Test a Laptop’s Cooling Fan [HP G42 – error 90B]


I own a 1 yr old HP G42 – 368 TX laptop. Everything was working smoothly until I booted it up and this error message appeared. It says that the system fan is not running / malfunctioning with an error code of 90B. I can log-in and access my operating system but I don’t go beyond 15-20 minutes because I don’t want to risk frying my processor.

I done the following steps: (You could try them out too.)

  1. Updated my BIOS. – failed
  2. Performed a power drain by removing the AC adapter and battery then pressing the power button for 1 minute. – failed
  3. Tried to run multiple processes on the operating system for 5 minutes. I was trying to force the processor to send a control signal to the fan to run. Be careful when you do this because it may risk frying internal parts.failed
  4. Blowing air into the fan while booting. Something like pushing a car while trying to start it. – failed

What I want is to isolate what and where the problem is coming from:

  1. Is the cooling fan still operating?
  2. Is the motherboard supplying enough DC to power my fan?

For the first item, I need to take out my laptop fan and use an external DC power supply to test if it is still working.

How test your laptop cooling fan:

1. Take out the fan. Disassembling your laptop is very tricky but I found this comprehensive step-by-step guide for a first time disassembler like me. Here’s the link.

2. Make a DC power supply. You can buy a switching power supply of up to 9 or 12V from any hardware store. This converts your wall socket’s 220V AC into DC. A laptop fan can be powered by 3 to 5V of DC power. Here’s mine:


3. Splice the end part of the power supply. Note that it has 2 wires – one black and one white. The black wire is the negative/ground terminal while the white is the positive terminal. Make 2 alligator wire clips – one red (for positive) and one black (for negative). Connect the red clip to the white wire and black clip to the black wire.


4. Poke a small copper wire in the red and black terminals of the fan. Splice the copper wire sideways in order for it to fit in the hole of the DC fan’s terminals. The DC fan has 3 terminals (red,black and yellow). The red and black terminals are the positive and negative DC power terminals respectively. The yellow terminal is for control.


5. Connect the alligator clips to the their corresponding colors. Turn the knob of the power supply into 3V before connecting it to the wall outlet. Run another test with 4V and finally 5V.


My case was a successful one because the fan was running smoothly. Now I could prove that the problem was that the motherboard was not supplying enough power to my fan. I ran a few voltage tests in the terminals of the motherboard and found that it was just 1V. I just have to isolate where the problem is.

I’ll write another post about this once I found the problem.


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About freezingrain17

I'm just like every other guy who prefers to do stuff on their own. My goal is to blog about my experiences as a troubleshooter to help other DIY people out there.

9 responses to “How to Test a Laptop’s Cooling Fan [HP G42 – error 90B]”

  1. 000 says :

    Hey, tranks for the information. Could you tell me the specific name/number of the fan?… I need to replace it too.

  2. Jonathan Moura says :

    Hello, i’m from brazil, and i have the same problem about the fan, error 90b, I’m using a hp g42 321br, and i tried all, since your arguments and others, i Believe who the motherboard don’t is powering the fan because i made the first test and the fan worked,but now i don’t know what i do, you solve your problem? please, can you help me, think is a problem of one sector of the motherboard. I Have hope you answer me, Bye.

    • 000 says :

      Have you checked if your BIOS is updated?

    • freezingrain17 says :

      hello jonathan, yes the problem is in the motherboard. I couldn’t get the laptop to detect the fan.

      What I did is connect the fan to another dc power supply in the board. The drawback of this is that the fan is “always on”. This setup will consume larger battery power unlike the one which has “control” (the motherboard will turn on the fan when the threshold temperature is reached).

      Although I still get the error 90B I can now use my laptop without the fear of overheating the board

  3. Pencil Portrait Lessons says :

    Good way of telling, and fastidious article to get facts regarding my presentation subject matter, which
    i am going to convey in college.

  4. Extremo41 says :

    How do you update the BIOS?

  5. I. says :

    Hi! I guess, I’ll leave this post for posterity without any hope for answer…

    I have the same 90b message on HP G56-108SA. I cleaned it and changed the thermocompund — all the same.

    It’s wierd, though, that after you turn it on and the error appears, the cooler is working fine, but when the Windows starts, it goes off. But when you push a small piece of wire through the grate, it is possible to start the cooler again. And it works fine until several reboots later.

    So, is it the cooler or the motherboard in my case?

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