There are multiple signs that should scream out to get your air-con serviced. If your air-con is blowing out warm air and not cooling as it should be then that is a sign that your system needs to be re-gassed. Warm air can mean leaking gas, which will be measure by air pressure during your service.
Your air-con isn’t going to smell like sweets and cakes, however there are certain smells you should look out for that can indicate your air-con has an issue. If you can smell a musty smell, your air-con probably has mold in it and contaminated with bacteria.
You air-con will always be a little bit noisy, however it can often be looked past when it is too loud or making noises it shouldn’t be. Loud bangs or rattling noises can mean that there or a worm out fan belt or there is debris clogging up the system filters.