In most of cases, it caused by one cartridge of the two was damaged, heating cartridges are aging (poor heating efficiency).
a. Take extruder assembly off, and unplug J2 & J9 in R interconnection board, measure
the resistance of the 2 heating cartridges. If it’s higher than 11Ω, replace it.
b. Replace R-nozzle temperature sensor and its cable.
c. Replace R interconnection board