My HP 840 turns on but after 1-2 seconds turns off.

Look at Step 2. The CMOS battery is encased in yellow. Just disconnect its connector by sliding it up and off. Press your power button for about 15 seconds to discharge any power in the system.

Reconnect the CMOS "coin" battery, plug in charger and see if it turns and stays on.

Let us know how it goes.

mike 의 2021년 1월 16일

@aactech I did a CMOS reset but it not helped me.

Ilya Liberman 의 2021년 1월 17일

@Ilya Liberman

Always disconnect power and remove the battery before doing anything inside. Next I would take out, one at a time, the hard drive and Wi-Fi card, to see if they are defective and causing the shutdown problem. If that doesn't fix your problem then try removing the RAM and reinserting. If two RAM modules try one and then the other.

mike 의 2021년 1월 17일

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Albert @v3nuo

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It could be the battery or the motherboard of your laptop. Remove the battery then plug in just the charger and turn it on see if it will replicate the problem. If it turns on that means it's the battery. Also check if it's your HDD to test if it's your HDD.

- Remove the HDD off the laptop then turn it on.

- You'll get an error message saying boot device is missing or something and that's normal

- See if it will turn off again without the HDD, if it turns off again that means it's the motherboard.

Replace a 4450 mAh battery compatible with HP Model CM03. Part #716723-271, 717376-001, CM03, CM03XL, CO06, CO06XL, E7U24AA, HSTNN-DB4Q, HSTNN-DB4R, HSTNN-IB4R. 48 Watt Hours (Wh). 11.1 Volts (V). 이미지

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HP CM03 Laptop Battery

$34.99

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So it works if the battery is disconnected and just the power adapter connected, is that correct? — mike
@Ilya Liberman

So it is probably a mother board issue. Something is overheating quite quickly. Here is the url for the HP service manual:

https://support.hp.com/nz-en/product/hp-...

One other thing. Have you tried resetting the BIOS?

HP Elitebook 840 G1 Motherboard Replacement

Unplug the charger and disconnect main battery.

Look at Step 2. The CMOS battery is encased in yellow. Just disconnect its connector by sliding it up and off. Press your power button for about 15 seconds to discharge any power in the system.

Reconnect the CMOS "coin" battery, plug in charger and see if it turns and stays on.

Let us know how it goes. — mike
@Ilya Liberman

Always disconnect power and remove the battery before doing anything inside. Next I would take out, one at a time, the hard drive and Wi-Fi card, to see if they are defective and causing the shutdown problem. If that doesn't fix your problem then try removing the RAM and reinserting. If two RAM modules try one and then the other. — mike