There are 3 sensors in the cartridge holder. The card reader at he top left, the pack end sensor, which tell the machine when the cartridge is empty, on the bottom right, and then the sensor that tells the machine if a cartridge is simply present at the very back. The cartridge sensor is a tiny little button that gets pushed in when the cartridge is in. If it doesn't get pushed, the machine rejects anything that the reader reads. Make sure that button is in place and able to be pushed. Other than that, the only other part that makes the machine read the cartridges is the heater control board which is $$$.