For the record - these older calculators are made to be taken apart, maintenanced and fixed. Unlike modern broken obsolescence machines. I love that people actually have the time to post an answer to a fix that says "I have no idea how to fix - buy a new one."
I have a Canon MP250 and had the same problem with keys sticking. I looked on the back of the machine and there was only 2 screws to take off and even if you removed the screws the back does not come off. So I started looking at the front and the keys - and therein lies the solution.
Each of the keys pops off with a flathead screwdriver when you lift on the left side of each key. I recommend only taking 5 or 6 off at a time. Clean with q-tips and alcohol both on the base of the machine and the actual keys. Spray with canned air to remove any other debris. Then reattach - snap each key back on - they only go on in the correct direction. After you have reattached the keys then test the machine. I had an extra problem with the PLUS + key - it still was sticking after the cleaning. So I stared at the base under the PLUS + key and saw 2 small metal pins that needed to be moved to the left with a flathead before you reattached the key. Once that was on correctly everything worked like brand new.
Here is a link to one of my earlier posts where I show how to test proper operation of the calculator. I invite you to read it.
One adjustment is important before we get started. By default the 12C displays only two decimal places. This is not enough. Personally, I like to see five decimal places, but you may prefer some other number. To change the display, press "F" and then the number of decimal places you wish to display. I would press "F 5 " to display 5 decimal places. That's it, the calculator is ready to go.
and 
keys; keep both keys held down for the next step.
key so the 
and keys are held down simultaneously. key. key, followed by the key. The main test screen displays the following: '1.L 2.C 3.H'.
to perform the LCD test (all LCD segments will be turned on). Press any key to exit.
to perform the checksum test and see the copyright messages. Press any key to go from one screen to the next until the main test screen, '1.L 2.C 3.H', is displayed again.
to perform the keyboard test. Press EVERY key on the keyboard until all the keys have been pressed at least once. The screen will progressively turn off with each key press. Once all the keys have been pressed and the screen is clear, press any key to return to the test screen. to exit the test system. This will also turn the calculator off.
If the calculator detects an error at any point, it will display an error message.
For the HP 12c financial calculators with one battery, to perform the LCD self test:
key, and simultaneously press and hold the key. then .
920 views
Usually answered in minutes!
×