A paperclip reset can be performed on all HP 12C calculators with serial numbers
before CN11500001. This will include serial numbers that start with numbers or with
other letters. These calculators will have three 1.5 volt button cell batteries. Follow
these steps for calculators in this serial number range:
1. Remove the battery door.
2. Take the batteries out.
3. Take a paper clip and bend it out enough to cross over from one terminal to
the other.
4. Hold the paper clip across the battery terminals for approximately ten
seconds.
5. Replace with a fresh set of batteries, being sure to follow the polarity
assigned. Note the picture beside the battery cover.
6. Replace battery door.
7. Turn on the calculator. The display should read "Pr Error". Press and release
the ON key twice.
Coin reset
A coin reset can be performed on all HP 12C calculators with serial numbers after
CN11500001. These calculators will have one CR2032 battery.
1. Remove the battery door.
2. Tap the calculator gently into your palm to remove the battery.
3. Place a small battery-sized coin (penny or a dime US) flat into the battery
compartment. Hold the coin in place with your finger.
4. While holding the coin, turn the calculator over, press the ON key, and hold for
10 seconds.
5. Remove the coin.
6. Replace the battery, being sure to follow the polarity assigned. Note the
picture on the calculator.
7. Replace the battery door.
8. Turn on the calculator. The Display should read “Pr Error”. Press and
release ON twice.
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