PC Boot Diagnostics: CMOS Checksum Error at Startup You see a “CMOS checksum error” during boot. What is the most likely underlying cause in typical desktop systems?

Introduction / Context:
The CMOS RAM stores system configuration (time/date, boot order, device settings) and is held by a coin-cell battery when power is off. The BIOS compares a computed checksum of CMOS contents with a stored value. A mismatch flags a CMOS checksum error during POST. Identifying the most common cause is essential for fast field repairs.

Given Data / Assumptions:

• Standard ATX desktop board with a CR2032 battery.
• System has been off or in storage for long periods.
• No recent firmware flashing errors.

Concept / Approach:

Over time, the CMOS coin-cell voltage drops. Low voltage leads to corrupted or reset CMOS contents, producing a checksum mismatch. Replacing the battery and re-entering BIOS settings usually resolves the error. While other faults can cause checksum errors, battery depletion is by far the most common in otherwise healthy systems.

Step-by-Step Solution:

Verification / Alternative check:

Measure battery voltage with a DMM; anything well below nominal 3.0 V indicates replacement. If errors persist, inspect for BIOS corruption or board faults.

Why Other Options Are Wrong:

Bad power supply can cause random issues, but the typical, isolated “checksum error” points to the battery. A BIOS update is not inherently required. Incorrect drive types cause boot failures, not checksum mismatches. “None of the above” is unnecessary given the common root cause.

Common Pitfalls:

Replacing the motherboard prematurely, or forgetting to reset time/date after battery swap. Also, mixing up CMOS (settings) with BIOS firmware code stored in flash.

Final Answer:

CMOS battery is nearing end of life