CMOS Checksum error 502 persists

[Vyper626]

I replaced the main battery a few months ago and now I keep getting the CMOS checksum error 502 every time I turn on my laptop. I have reset and updated the BIOS more than once.  I figured the CMOS battery was bad, but couldn't find it. After some research, I found out that my laptop didn't have one or rather, it was part of the main battery now (which in my own opinion is a really dumb move). Also, the laptop will not turn on unless it is plugged in. Once I plug it in and turn it on, I can unplug it again. It will show the 502 error and then load Windows, but it's getting annoying having to plug my laptop in just to turn it on. I have tried just about everything I could think of/research.

[VikramTheGreat · Accepted solution]

Hi @Vyper626,

Welcome to the HP Support Community.
 

Thank you for posting your query. I will be glad to help you.

The CMOS Checksum Error 502, combined with the need to plug in the laptop to power it on, strongly suggests that the CMOS battery function (now integrated into the main battery) isn’t working properly, even though the main battery itself was replaced.

What You Can Try

1. Full Power Reset

Sometimes residual power causes CMOS issues:

1. Turn off the laptop.
2. Unplug the charger.
3. Hold the power button for 30 seconds.
4. Plug in the charger and try powering on.

2. Re-seat or Replace the Main Battery Again

Even new batteries can be defective or improperly seated:

• If you’re comfortable opening the laptop, try disconnecting and reconnecting the battery.
• If the battery is third-party, consider replacing it with a genuine HP battery.

3. Clear CMOS via Jumper or Internal Reset

Some HP models have a CMOS reset pin or jumper on the motherboard:

• This requires opening the laptop and shorting the pins briefly.
• If you’d like, I can help you find the service manual for your exact model.

4. Check for BIOS Recovery Option

• Power on and press Windows + B and then Power (hold for 2–3 seconds).
• This may trigger BIOS recovery if the checksum error is due to corruption.

🔗 HP Support Reference

👉 HP Desktop PCs - Resolve BIOS ROM or CMOS checksum error | HP® Support

I hope this helps.

Take care and have an amazing day!
 

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Regards,

VikramTheGreat

[Vyper626]

Thank you for responding. The only suggestion that I have not tried yet is the 3rd option (jumper/internal reset). I'm saving replacing the battery as a last resort. I have an HP 15 da0037wm laptop that has been upgraded to Windows 11