It has just come to our attention that canon printers that are sold with the 37 black and 38 colour low capacity cartridges should be able to use the 40 black and 41 colour high capacity cartridges. They are the same size but if you remove the lid of the low capacity one you find that half of the internal volume is used up by a huge lid. Many of our customers have returned either the black or colour XL cartridges claiming that they are being rejected by the printer and we have obliged with a refund in all cases.
Today a customer brought her printer to us and we observed that the printer did indeed reject the cartridge but, and here is the shocker, only if you mix the low and hi capacity cartridges. When we replaced both the low capacity cartridges with high capacity ones they worked!