What Does This Error Mean?

A Casio calculator that resets to factory settings after a battery change is normal — the main battery also maintains memory on most Casio models, so removing it erases stored data. To prevent this on graphing models (fx-9750, fx-CG50), install the new battery quickly without leaving the battery compartment empty for more than a few seconds.

Affected Models

Casio fx-9750GIII
Casio fx-CG50
Casio fx-9860GIII
Casio fx-991EX — STAT and program memory
Casio ClassWiz — program memory

Common Causes

Battery removed and left out — no backup power to retain memory
Battery completely dead before replacement — all retained RAM lost
Full reset accidentally performed (SHIFT + 9 > All Reset)
Battery contacts corroded, intermittently losing power and triggering reset
Capacitor backup power exhausted on older models

How to Fix It

Change the battery quickly.

On most Casio calculators, the main battery also supplies RAM power. To minimise memory loss, have the new battery ready before removing the old one. Swap batteries as fast as possible — within 30 seconds is usually sufficient to retain data on most models.
Back up programs before battery changes.

On Casio graphing calculators (fx-CG50, fx-9750GIII), use the Casio FA-CP1 USB cable and Casio's FA-CP1 software or fx-CG Manager to back up programs to a PC before changing the battery. This is the only way to reliably preserve custom programs and data through battery maintenance.
Check battery contacts for corrosion.

If the calculator resets randomly without a battery change, the battery contacts may be corroded, causing momentary power loss. Open the battery compartment and look for white or greenish residue on the contacts. Clean with a cotton swab dipped in white vinegar or isopropyl alcohol, then dry completely.
Re-enter your settings after a reset.

After a factory reset, the calculator is in COMP mode, Degrees, and Norm 1 display format. If you need different settings (Radians, Fix mode, etc.), go to SHIFT + MENU (Setup) and reconfigure. For STAT data, it will need to be re-entered manually.
Use fresh, high-quality batteries.

A low-quality or near-dead battery that suddenly drops below the minimum operating voltage can trigger an unexpected reset even without physically removing the battery. Use brand-name CR2032 or AAA batteries from known manufacturers. Replace batteries proactively at the start of each exam period rather than when the low battery indicator appears.

When to Call a Professional

Settings loss after battery change is a design characteristic, not a fault. No professional service is needed.