This error could be caused by dirt on the encoder strip or on the rod on which the cartridge carrier rides, the encoder strip is out of position or a faulty cartridge (one or both).
First try cleaning the travel rod. Turn off the printer and unplug it. (I recommend unplugging the power cord from the printer and then from the wall outlet.) Open the front cover to access the ink cartridge area. With a tissue or paper towel, clean off the exposed section of the metal rod that the cartridge carrier runs on. Pinch the cartridge belt and slide the cartridge carrier over to the other side and clean the rest of the rod. Then slide the cartridge carrier back to the home position. (The tissue will end up covered with ink so be careful about getting ink everywhere.)
Also be careful while doing this cleaning step to avoid bumping the clear encoder strip. If this strip is dislodged from the cartridge carrier, you will need to slide it back into position. This entails carefully pushing it down and then back up along the back to the cartridge carrier so that it fits in the slot in the carrier. Sometimes you need to also move it forward and back before moving it into the slot. Close the front cover and plug the printer back in. Then turn it on and see if the error persists.
Next try cleaning the encoder strip (it is plastic). Use the same steps as above (turn off, unplug, and open front cover). Then move the cartridge carrier to the center (pinch the belt again) and remove the cartridges. Then move the cartridge carrier to the left.With a soft cloth clean the exposed section of the encoder strip, then move the cartridge carrier to the right and clean the rest of the strip. Get the encoder strip back into position if it is out of place. (See above for the details.) Replace the ink cartridges (after moving the carrier to the center again). Slide the cartridge carrier back to the right, close the printer cover and plug it in. Turn on the printer and see if the error persists.Work quickly - the cartridges should not be out of the printer for more than 30 minutes at a time.
For pictures of the encoder strip, see http://support.lexmark.com/index?page=content&id=SO3370&actp=search&viewlocale=es_ES&userlocale=ES_ES&segment=SUPPORTproductCode%3D&productCode=LEXMARK_X6150&searchid=1284653009138
Next try trouble-shooting the cartridges. Remove one cartridge at a time with the unit unplugged. Then plug it back in and turn it on. Press Copy, then press Options 10 times (or until you see Maintenance on the LCD screen). Press the Left arrow button once (it should read Self-test Page). Then press Select. The test page should print. Replace the first cartridge and repeat these steps with the second cartridge removed. Also do the self-test with both cartridges in place.
Replace the cartridge(s) if the problem is with one of them.
Contact Lexmark for repair if the printer is still under warranty and none of the above steps help. Otherwise, decide if a repair is worthwhile or if replacing the printer is a better option. (It's a minimum of $50 just for the service center to diagnose the problem plus the cost of shipping the printer.)
I hope this helps.