Canon Pixma MX870
My wife had a mishap and snapped off the front paper door with the sliding closet door. I could not force it into the case to replace it although it looked undamaged. My wife however does not take no for an answer and somehow forced it back into place. It is now permanently open.

Experienced technical support provide their insights in answering this question :
Kind of hard to advise without seeing exactly what happened, if anything broke, etc.

How to Identify and Fix Common printer Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced technical support :

Canon PIXMA MX870 Wireless Office All-in-One Printer (4206B002) (Renewed)
(1) Open the Paper Support, raise it, then tip it back. (2) Open the Paper Output Tray gently, and open the Output Tray Extension. (3) Slide the Paper Guides (A) to open them and load the envelopes in the center of the Rear Tray WITH THE ADDRESS SIDE FACING YOU.
Despite its sporadic hiccups in output speed, we recommend the $199 Pixma MX870 as a worthwhile printer for shoppers that need a multifunctional printer at a very reasonable cost.
Features. The Canon Pixma MX870 Office All-In-One Printer wirelessly brings professional-quality printing, copying, scanning, and faxing to your small business or home office. This Pixma is ready for networking from within range of your wireless router or you can network via its wired Ethernet interface.
A standard C4 envelope measures 324mm x 229mm. This is designed to fit unfolded A4 paper.
Press [Adjustment/Maintenance] → [Adjust Action] → [Adjust Fold Position]. Select the method for the folding position of paper.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

My printer feeds through one piece of paper all the way and then a half piece of paper and stops. I pull out the paper to stop the paper jam and then it prints but sounds rough. After printing, it doesn't feed the paper all the way through.
ANSWER : And the first 2 sheets, the full and half sheet that go through first, are blank.

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Canon Pixma MX870

Front paper door replacement
ANSWER : Kind of hard to advise without seeing exactly what happened, if anything broke, etc.

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Canon Pixma MX870

Front panel under front door (holds developer & toner installation area)
ANSWER : Not familiar with that model, but all doors have a peg on each side that go into holes in the frame. Did you try putting one peg in and then flexing the door to get the other one in. Without seeing it, that’s the best I can offer.

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Canon Pixma MX870

Canon Pixma Printer Paper Does Not Feed Properly/"No Paper" Error Occurs?
ANSWER : Pretty much useless advice. I also have serious doubts about someone who is obviously not fluent in the English language.

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Canon Pixma MX870

Error Message "Paper out" when paper loaded
ANSWER : Check your paper tray settings as printer may be looking for paper from the manual feed tray on top of printer.

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Canon Pixma MX870

paper drive wont accept paper
ANSWER : There is probably a small scrap of paper stuck at the left edge of the paper path. The paper hits it and skews to the left. The smaller size bypasses it. You need to shove something stiff, like a piece of file folder, down the paper path and attempt to dislodge it.

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Canon Pixma MX870

Error message to load paper and push OK, but not out of paper
ANSWER : I assist the binary options for a long time, and only half this past year I decided to alter the strategy for a better, she called for help Bing and he gave me this web site http://bestoptionstrategies.net .Now I focus on the suitable technique for themselves, and things gone up. I think everybody else faced with this particular problem. I believe my estimation will soon be helpful to you.

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Canon Pixma MX870

when printing front and back, the front will print well but the back will fade
ANSWER : It happens because second side voltage is much higher and the board breaks. Are you getting this issue in auto duplex only, or same error you are getting when you print second side manually?

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Canon Pixma MX870