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If you set your thermostat for, say, 69 degrees on your Fedders air conditioner, but you notice the room temperature is too warm or too cold, chances are you need to inspect your thermostat.
As with any air conditioning part, you'll need to open the box and clean everything inside. Wipe away any dirt, dust or accumulated grime. Next, check for corrosion on all the internal parts. You may be able to clean off the corrosion you find, but if it is too far gone, consider upgrading with GE appliance parts, Kenmore appliance parts, or whichever model you happen to own.
A mechanical thermostat should be level. These are little containers with a blob of mercury that makes an electrical contact when the temperature is right. This should be level.
You can check it with a small "bubble level" which is basically a ruler with a bubble that shows you when something is exactly horizontal. The bubble will rest in the center of the window. If the thermostat is not level, fix it by gently pushing it up or down until it is level.
If these tips do not work, you may need to get a new thermostat. If you do need to replace the thermostat, check with your local appliance part depot to see if they have your make and model in stock.