I have an Yihua 937D Hakko clone that started reading E-5 on the digital temp readout. It would stop heating but could be fixed by twisting the grip which led me to think it had a bad solder joint in the wand but could not locate the problem. Then last night it quit altogether(still read E-5) but no amount of wiggling or cursing would generate heat so I dug out my old 20w iron and went over some suspicious solder joints and now all is well. The original solder had wetted but I don’t think heat had been applied long enough to sufficiently heat the wires as they looked “in” the solder but not “one with” the solder. Just a drop of flux and some heat was all it took so don’t ditch your old iron.
Well done Rufusbduck. I think its about time you started modding it to get a few more watts out of it. 
Flux makes everything better. :p