HP computer won't power up after CMOS reset

[Harrytech-30]

Hello Community,

My HP EliteBook 840 G5 gives off flashing light codes on the caps lock and numlock keys upon restarting. Afterwards, the cooling fan will run up to full speed with the display still off. After several hard resets, the system boots up normally.

I had tried to diagnose the issue by doing a memory test which it passed, I also did a SMART test on the SSD which also passed. The issue still continued with failure to boot. I suspected a malfunctioning RAM or BIOS, I started with taking out one of the RAM cards to check and the computer booted up normally, I did same action with the second RAM, the computer also booted normally, i concluded it was not the RAM. I attempted a BIOS reset by pulling the CMOS battery and the performing a current drain by holding the power buttion ON for 30 seconds, I did this with the main laptop battery disconnected. AFter the hard reset the PC has failed to startup. I can see the charging light come up and after holding the power button ON for 15 secs, the caps lock light comes up for 5 seconds and goes off.

Please what action can i take to fix this issue?

[BongChaGomez]

Hello @Harrytech-30,

Welcome to HP Support Community.

We are sorry for the trouble that you are going through. You’re dealing with a classic EliteBook G5 firmware/embedded controller failure scenario, and the behavior you describe (LED blink codes, fan at full speed, no display, now not POSTing at all) is not RAM or SSD related at this point.

I’ll break this down clearly and then give you exact recovery actions, in order of likelihood to work.

What’s actually happening

On HP EliteBook 840 G5, the following symptoms together are significant:

• Caps Lock / Num Lock blinking on restart

• Fan ramping to full speed with no display

• Boots only after multiple hard resets

• After CMOS removal → now no POST at all

• Power LED + brief Caps Lock light, then nothing

This strongly points to BIOS corruption or Embedded Controller (EC) firmware lock-up, not hardware RAM failure.

First: Identify the blink code (if possible)

If you can reproduce it again:

• Count Caps Lock blinks

• Count Num Lock blinks

• Note if they blink together or alternately

Typical meanings on G5 series:

• 2–3 blinks → BIOS corruption

• 5 blinks → system board / EC issue

• Continuous fan + no display → BIOS recovery mode failing

Even if you can’t get the blink pattern now, the next steps still apply.

Step-by-step recovery actions (DO IN THIS ORDER)

1️⃣ Perform a proper HP EC reset (very specific)

You already did something similar, but do it exactly like this:

1. Disconnect:

   • AC adapter

   • Internal battery

   • CMOS battery

2. Remove ALL RAM

3. Hold Power button for 60 seconds (not 30)

4. Wait 10 minutes (important – EC discharge)

5. Reinstall:

   • CMOS battery

   • Internal battery

   • ONE RAM stick only (slot closest to motherboard)

6. Plug in AC (do NOT insert second RAM yet)

7. Power on and wait up to 2 minutes

If it boots → immediately update BIOS later
If no display → continue

2️⃣ Force HP BIOS Recovery (Crisis Recovery)

This is the most important step now.

What you need

• Another working Windows PC

• USB flash drive (8–16 GB, FAT32)

Create BIOS recovery USB

1. Go to HP Support

2. Download BIOS for EliteBook 840 G5

3. Run the BIOS EXE

4. When prompted, choose “Create Recovery USB”

Perform recovery

1. Insert USB into the EliteBook

2. Hold Windows key + B

3. While holding keys, press Power for 2–3 seconds

4. Keep holding Win + B for 15–20 seconds

5. Release and wait

What to expect:

• Fan spins

• USB activity light blinks

• After 30–90 seconds, screen may turn on

• BIOS recovery screen appears

⚠️ Even if screen stays black, wait at least 5 minutes before giving up.

If Win + B doesn’t work → repeat using Win + V

3️⃣ If still dead: test without CMOS battery

Some G5 units recover only this way:

1. Remove CMOS battery again

2. Leave it out

3. Install:

   • Internal battery

   • One RAM stick

4. Try powering on

If it POSTs:

• Shut down

• Reinstall CMOS battery later

When it does boot again (important)

Immediately:

1. Update BIOS to latest stable version

2. Enter BIOS → Load Setup Defaults

3. Disable:

   • Fast Boot

   • Modern Standby (if present)

4. Check BIOS event logs for firmware errors

Please let us know the output and hope this helps us resolve the issue.