Aircon wall mounted, sometimes working, sometimes not.what is the possible cause?

"sometimes working", assuming you mean works normally in terms of air temperature, eliminates a couple of the suggestions in the post above. You won't be low on freon, and it's less likely to be a thermostat problem (although it could still be).

Best first bet would be a bad starter capacitor for the compressor motor. Relatively inexpensive to replace, so well worth a try if the following symptom sounds familiar: As the capacitor starts to fail, the compressor won't always move when the thermostat calls for cool. It'll just sit there stalled and pop the overload. You'll hear the thing "hum" for a bit, then with a small "pop" (that's the overload kicking out), and the "hum" goes away. This may continue to cycle this way indefinitely.

Another possibility -- are you running the A/C heavily in a very humid climate? If so, you might want to check to assure your coils aren't completely coated in ice. The lack of air flow will cause the unit to operate extremely inefficiently, heating things up to produce pretty much the same symptoms I described above.

Those are the two inexpensive problems / solutions. After that, more advanced troubleshooting (and more $) are involved.
Low refer gas, loose electrical connection, bad thermostat and a dozen more, call the man pay the bill. Nothing there you could not make worse.

The answers post by the user, for information only, does not guarantee the right.

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