I gradually loosened up the 5 screws, which hold the battery, until it worked like a charm. Redid it 2 times the same week because I was stingy on screwdriver turns.
When problem occured:
Charging cycles: 150
29 months old
Now:
Charging cycles: 212
34 months old refurbished European 13 ‘’ MBA early 2015 A1466 (128 GB SSD, 1,6 GB proc., 8GB RAM)
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=== Update (12/08/2019) ===
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Issue reoccured about a month ago, disappeared and reappeared at its own will, after one week the issue was permanent.
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No method I tried worked (loosening battery screws, pushing multiple / different keys / the keyboard hard at different locations, putting something below the motherboard to lift it slightly, removing the battery, removing keycaps that have a metal hinge part, using pressurised air without and with keys being removed).
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Thinking about the answer below from centar07 I disconnected the two cables that connect the keyboard to the motherboard.
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On the plug of the cable connecting the keyboard to the trackpad I found well visible soft white solid stuff on 2 pins on the far side where the tab button is.
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I removed it in one second using my fingernail and the keyboard worked instantly and has been working since.
I gradually loosened up the 5 screws, which hold the battery, until it worked like a charm. Redid it 2 times the same week because I was stingy on screwdriver turns.
When problem occured:
Charging cycles: 150
29 months old
Now:
Charging cycles: 212
34 months old refurbished European 13 ‘’ MBA early 2015 A1466 (128 GB SSD, 1,6 GB proc., 8GB RAM)