I'm sure most of you have come across cmos checksum error, prompting you with this annoying screen below

Whenever we get this error message we generally assume that the cmos battery is weak and need replacement. This is what we experienced when we were troubleshooting one of the machines brought to us by our client. The machine keep on bugging us with this error message whenever it boots. We've tried replacing the battery, but the results are the same.

We finally did a component level checking nearby the cmos section, apparently the capacitor looks fine with no leakages, but when it's tested using the capacitor metre it shows us a lower reading than the usual. Electrolitic capacitors have their own tolerance capacitance of -10 percent, +20 percent. For instant the capacitor close to the cmos found on this board is 47uf, so the tolerance capacitace should be between 42.3 to 56.4, but strangely we are getting a reading of  39.2 which is below the tolerance range. That explains why the annoying messages keeps popping up because the capacitor which stores up voltage is not getting sufficient juice to supply the current to the CMos chip.

Capacitor metre being used to measure capacitance tolerance

                                                                                                                                               Intel CMOS chip
 

For motherboards if you can't get the exact capacitor with the same microfarad as a replacement, you can always use a capacitor with a higher microfarad provided the voltage is the same.

Once we have replaced the capacitor, the machine boots up smoothly with no error messages.