Ac model # 42sd6b er-05 code meaning

Are you trying to run your cooling now? I'm guessing you have a carrier thermostat and that's where this is displaying. It's been awhile since I've came across one but to my vague reculiction that code indicates the outdoor sensor detects low ambient them temperatures which in turn will not allow your air conditioner to run which would make sense this time of year. If you want to run cooling in below 60 degrees temps you must add a "low ambient kit" this shuts the fan down outside to build up higher head pressure without doing this you will just ruin your ac but in your case it won't let you. Google up your Tstats model number and codes if that's not your situation.