CMOS 502 checksum error

[T688]

I cannot turn on my OMEN 25L unless I remove battery, I always get 502 checksum error, please help. I have updated BIOS to the last version and changed battery. I can only sleep computer if I shut down I cannot start, I have to remove battery again. I have tried to set BIOS defaults also. PLEASE HELP!!

[Hawks_Eye]

Hi @T688 

Welcome to the HP Support Community! We're here to help you get back up and running.

I understand the concern. The CMOS 502 checksum error on your OMEN 25L is linked to BIOS data not being read correctly. Since you have already updated the BIOS and replaced the battery, the next step is to carefully clear CMOS and re‑initialize BIOS settings using HP’s structured process.

Perform a Full Power Reset

This helps discharge residual power and reset hardware states.

Disconnect the power cable from the desktop

Press and hold the power button for 20–30 seconds

Reconnect the power cable and try powering on

 

Clear CMOS Settings

Resetting CMOS ensures BIOS settings are refreshed.

Shut down and unplug the desktop

Open the side panel carefully

Locate the CMOS battery on the motherboard

Remove the battery for 5 minutes, then reinsert firmly

Close the panel and reconnect power

 

Load BIOS Defaults

Ensures BIOS settings are restored to stable defaults.

Power on the system and press F10 to enter BIOS

Select Apply Defaults and Exit

Save changes and restart

 

Run HP PC Hardware Diagnostics

Verifies hardware integrity and detects issues.

Power on and press F2 repeatedly to open diagnostics

Run System Tests → Extensive Test

Allow the test to complete and note any error codes

 

Update BIOS and Chipset Drivers

Ensures firmware and drivers are aligned with hardware.

Visit HP’s official OMEN 25L support page

Download and install the latest BIOS and chipset drivers

Restart the system after updates

Following these steps should stabilize the startup sequence and reduce the need to remove the battery each time. If the checksum error continues after clearing CMOS and updating BIOS, it may point to deeper motherboard RTC circuitry issues. Running diagnostics will confirm if the problem is isolated to CMOS or involves other hardware.

Take care, and have an amazing day!

Regards,

Hawks_Eye

[T688]

I have tried everything I always get checksum 502 error. I even changed the power supply. Can I uninstall the OMEN setup utility for old BIOS or downgrade BIOS version? I cannot power off the computer. If I power off I have to remove the battery from motherboard to be able to power on.

[Deep_World]

Hi @T688,

I understand how this must be, especially after already updating the BIOS, replacing the CMOS battery, changing the power supply, and performing all the recommended steps. I appreciate the time and effort you've invested in trying to resolve the issue.

Based on what you've described, since the CMOS Checksum 502 error continues to occur and the system will only power on after removing and reinstalling the motherboard battery, it is unlikely that the issue is related to a BIOS setting, the OMEN Setup Utility, or the power supply. While  about the BIOS downgrades We do not generally recommend downgrading the BIOS, as it may not resolve hardware-related faults and can introduce additional issues.

The symptoms suggest there may be an hardware issue involving the motherboard, CMOS/RTC circuitry, or another board-level component that is preventing BIOS settings from being retained correctly.

At this stage, To ensure your device receives the appropriate care, we recommend visiting an authorized HP service center. Our certified technicians can provide a thorough diagnosis and offer the best solutions to address the issue.​

You can locate the nearest HP service center using our Service Center Locator:​

🔗 HP Service Center Locator

If you need further assistance or have any questions, please don't hesitate to contact us.​

Regards,

Deep_World