CMOS Checksum Bad error

A CMOS check sum error is usually caused by corrupted data in the CMOS memory. When the computer boots it adds all of the ones and zeros in the CMOS and drops the carry bit. It compares the resulting sum to a like (we hope) sum that was generated when the CMOS data was produced. If the two do not match, the CMOS is presumed corrupted, and an error is generated. The corruption can be caused by defective CMOS memory or power problems caused by a shorted motherboard, an expansion card that is not properly seated, a defective component, etc. I would check the battery with a multimeter and make sure it is good and installed correctly. Some older motherboards have a CMOS battery soldered onto the motherboard and have a jumper which must be changed to use an external battery. The battery on the motherboard should be removed if an external battery is installed. It can leak and damage the motherboard beyond repair. Do not use a soldering iron or anything hot to remove a battery form a motherboard. The battery can explode. I use a combination of pliers and diagonal cutters to remove these batteries. The computer must be unplugged when doing it and care must be employed (not an operation for an overly ambitious gorilla). Some batteries include the system clock, are usually black and rectangular, soldered to the motherboard, and are supposed to have a 10 year or greater lifetime. When they fail, the motherboard is usually a goner. Most motherboards have a jumper, which can be used to discharge (erase) the CMOS memory, and this may fix the problem. Be sure the computer is off and unplugged, move the jumper to the discharge position, wait 10 minutes, move it to the normal position, attach the power cord, and turn-on your computer. Some motherboards can be damaged if the computer is powered on with the jumper in the discharge position.