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So really the only thing you can do to start is see if the over temp light is accurate or not. Put a cheap thermometer inside the freezer and it should be around 0ºF in 3-4 hours or so. If it's somewhere above 15 degrees then you have a problem and you could have a freon leak, defrost issue, a bad inefficient compressor or something else. Otherwise if the temperature is reading properly, then the control board is just confused and you'll want to unplug the freezer for 5 minutes to see if it resets. If not, then the control board would need to be replaced to get rid of the light.
You can't check for one yourself. You must be licensed and you'd need specialty tools like a piercing valve, torches, and gauges to tap into the freon system to check the pressure. This will require a technician.
Hello again. I'm just following up because I haven't heard back from you and I see this question is still open. Is there anything else I can help you with?