Hello, Canon Pixma MP210, new out of box (although it had been sitting in the box for quite some time). First paper through went in crooked, and since then it powers up with an E23 error. Can't do a thing. No paper jam, no foreign object. Per other suggested solutions, I've already tried purposely creating a paper jam (to reset the error message) and also forcing a piece of cardstock through the rollers (to clear any unseen debris). No luck. What next?

Welcome to Fixya

E23 means Paper Feed error. Since you already checked that there is no foreign object stucked from the inside, try to reset the printer. See procedure below:

With the printer power OFF, press the Power button while pressing and holding the Stop/ Reset button. When the Power is ON, hit the Stop/Reset button 2 times while pressing and holding the Power button. When releasing the Power button and the Stop / Reset button, the printer will go into a service mode. When the ON-OFF lights in Green, hit the Stop/Reset button the specified number of times according to the function listed in below. Each time the Stop/Reset button is pressed, the LED lights in orange and green, starting with orange.

Hit 4 times for waste ink counter reset.

The number of the Reset button pressing LED
0 time = Green Power off
1 time = Orange Service pattern print
2 times = Green EEPROM print
3 times = Orange EEPROM reset (EEPROM reset)
4 times = Green Waste ink counter reset

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Try the following procedure for Error E23

1. Start with printer off
2. Press and hold resume Button (triangle in the circle) and click "Power" button. green led(Light) should be on
3. While holding the Power button down, release the Resume Button.
4. Without releasing the Power Button, click "Resume" button for two times and release Both Buttons(1.The power button 2.Resume button)
   
5. Click "Resume" for four times then soon click "Power" button, to confirm the selected action (reset counter absorber).
6. Restart the printer.