HP Deskjet 5150 - top cover open
I had the same problem, and the cause turned out to be the cover switch not being fully pushed in. On the underside right edge of the top cover, there's a little triangular protrusion that pushes down a black rectangular button (the cover switch) when the cover is closed. The problem occurs because the flimsy top cover is made of plastic and flexes slightly when you close it. This means that sometimes, the right edge of the cover won't seat properly even though it appears as if the cover is closed all the way. When this happens, the triangular protrusion isn't fully pressing the switch down.
To test if this is this is happening, send a print job to the printer and wait for the "top cover is open" error message to appear. Next, open the top cover door and locate the cover switch. It's a little black button in a rectangular hole at the right edge of the doorway near the ink cartridges. Take a pen or a car key and use it to push the cover switch down. You can also use your fingertip to do this if it's skinny enough. If you keep the cover switch pressed down, the power button should start blinking to indicate a print job is coming through. If the print job actually starts printing, then you know the problem was the cover switch.
In my case, all I had to do the fix the problem was close the cover and make sure it was fully seated by firmly pressing down the right edge of the cover until I heard a plasticky "click" noise. After that, the problem went away.
Hope this helps.
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