CPU struck at 0.39 GHz

Verify in BIOS settings to see if the power adapter is recognized.  It's one of the causes of CPU downclocked issue.

@Chino de Oro

The AC Adapter is recognised. The laptop comes with a 65W adaptor but supports 90W adaptor. I'm now using the 90W adaptor.

Additionally the problem first occurred in Red Hat Linux where the hardware issue is indicated in the Charge status LED. ( The light blinked 3 time in white color and 3 times orange color)​.

I removed, cleaned and remounted the RAM. That fixed the issue. But then the CPU struck issue happened.

Let perform an RTC reset.  It will reset the BIOS to factory default settings, resets the computer date and time, and unprovision Intel vPro.  Take note of your current settings if they are not using the defaults.

1. Turn off the computer.
2. Connect the AC adapter to the computer.
3. Press and hold the power button (around 30-35 seconds), until the power LED blinks three times.
4. Release the power button.
5. The RTC reset is complete after the power button is released.

You will need to adjust time, date, and other settings when reset is complete.

@Chino de Oro​ 

I did the RTC rest. No difference. Still at 0.39 GHz

What is your current BIOS version. When did the problems start. After BIOS update?

Latest BIOS version is 1.37.0, released 22 May 2024

https://www.dell.com/support/home/en-us/drivers/driversdetails?driverid=49w55&oscode=wt64a&productcode=latitude-14-7480-laptop

You can't just downgrade this latest version without special BIOS downgrade utility called "Bridge activation" tool.

Dell support can provide the tool for you.  After running the "Bridge activation" you can run the previous BIOS installation.

Here is few words from "Important information" of the BIOS update page:

Once the BIOS is upgraded, you cannot downgrade the BIOS EXE/Recovery (RCV) to earlier versions due to restrictions. BIOS downgrades are restricted when there are security updates and important fixes with earlier versions that impact the functionality of the system.

You do not have access to other RAM parts from working device?

How about update the latest BIOS back to device?

Do you have Dell docking station in use, if yes what is the model?

Test without dock.

Test without charger.

Remove all external USB devices etc. Use just the plain laptop.

@heikkuri​ 

I tried different versions of BIOS. That doesn't solve the issue.

I changed the value of intelppm from 3 to 4, now the laptop is running in 2.90 GHz. But it's slow as it's running in 0.39 GHz. 

And I again checked the RAM, cooling fan and SSD status. Everything is fine. 

The issue didn't first or occurred during a BIOS update. It first occurred in Red Hat Linux. It was 2 3 days after I changed the OS from windows 11 to Red hat Linux. The BIOS event log on the day says there is a issue due to storing the laptop in a hot environment.