Sorry if it is seated and capped then I don't have a quick fix.
The encoder strip and or the sensor are the most likely culprits... Have you cleaned the encoder strip? Did you use isopropyl alcohol and a swab?
Of course if you are still getting that error code, that needs to be addressed...
These are all things a tech could look at after he fixes the error code, and you have him there:
The cable tension, the cable needs replacing, the scan motor...\
You could also have a number of calibrations that need to be done. I did have the offset cap causing the heads to dry out but
due to my defective (and still not fixed ever) print cut alignment I have all the above changed and it now docs properly.
What is your test print like? How are all the sponges including the big one under the wipers? The scraper behind the wipers? Dampers? Caps?
Head alignment, print cut alignment...