Nozzle clog

Hi everyone,

I've had this KS1 for just under 2000 hours and its been great for the most part. I've recently been getting clogging error 11518. The start of the print looks great then turns to mush.

I've checked the nozzle, replaced the nozzle, done a system restore, increased nozzle temp, played with the tension cold pulls and opened the extruder with no broken or flaked filament.

The filament is extruding fine until I start a print. Does anyone have any suggestions?