I refilled HP 88 cartridge but printer (Officejet Pro L7590) still thinks cartridge is empty. How do I reset ink level?

Under the cartridge is a black like pump - on a new cartridge this is hard and rests firmly on an area inside your printer. As you print, the pump becomes softer - this is how the printer determines it is empty. Once refilled, the pump does not get back to the original state. Open the bottom of the cartridge and put a firm solid piece of folded paper in there - make sure it is steady and re-close cartridge. It will now read as full but may also show full - judge ink levels by the quality of print. Also make sure chip has not expired.

Did you replace the chip? There is a chip that records when the cartridge goes low and then empty. The chip needs to be replaced. Most likely you can set the printer to continue printing even though it is receiving an empty cartridge message but if you print and there is no ink head damage can occur.

As a former employee of HP in the ink labs, next time just buy a real HP ink cartridge. HP makes them so you can't easly refill and reuse them. When you do you also violate your printer's warranty and the product's lifetime phone tech support option. The 88XL inks are the least expensive was to go, don't be so cheap as to go and use cheap refilled inks. It's NOT worth it. P.S. Sorry, but yes, you will most likely rune your printheads too. The lesser quality inks of refills will clog up the printheads, and they're $85 each to replace. Again, it's NOT worth it being cheap with refilled inks.