What Does This Error Mean?

Casio calculator keys that do not respond are most often caused by the calculator being in a frozen software state, or by dirt and debris under the key contacts. Perform a full reset by holding the ON key or using the reset hole on the back. For physically sticky keys, a small amount of isopropyl alcohol on a cotton swab can restore contact.

Affected Models

Casio fx-991EX
Casio fx-82ES Plus
Casio fx-CG50
Casio fx-9750GIII
Casio ClassWiz fx-991CW
All Casio calculator models

Common Causes

Calculator in a frozen or locked state — reset required
Liquid spill under the keypad — residue on key contacts
Debris compressed under specific keys blocking the rubber contact dome
Calculator in a specific mode that blocks certain key inputs
Battery too low to register key presses reliably

How to Fix It

Perform a full reset.

Hold the ON key for 3 seconds, or locate the small reset hole on the back of the calculator and use a paper clip to press the reset button. Release, allow the calculator to restart, and test the keys. A reset clears software states that can lock the keypad.
Replace the battery.

A very low battery can cause unpredictable key response — some presses register, others do not. Replace the battery with a fresh one before assuming a hardware fault. Even a battery that appears to power the display may be too weak for reliable key sensing.
Check the mode — confirm keys are not locked.

Some Casio modes restrict key functions. For example, in STAT mode, certain keys open table menus rather than entering digits directly. Press MENU and select COMP (Computation) mode to return to the standard input mode and test the keys again.
Clean the keypad.

For keys that feel physically stiff or require extra force, dirt may be under the rubber dome. Dip a cotton swab in isopropyl alcohol (90%+) and gently press around the edges of the unresponsive key. Allow to dry completely before testing. Do not pour liquid on the keypad.
Test each key systematically.

Enter the Casio diagnostic mode if available for your model (varies — search your model's service manual). Otherwise, test each key by pressing it and checking the display. If only keys in one row or column fail, this indicates a matrix fault from liquid damage rather than individual key issues.

When to Call a Professional

If all keys on one row or column fail after a reset and battery replacement, the key matrix trace on the circuit board is likely damaged from liquid ingress and needs professional repair.