Check engine light codes

I own a 96 Grand Caravan and today when I started the van it felt "rumbly". A few minutes after starting the van the check engine light came
on. I followed the direction in the Haynes manual to run the ignition key code check. The codes I got were 12, 43, 31, 31 and 55. I know and understand what code 12 and 55 are, but am at a loss as to why I got code 31 twice and what exactly do these codes mean. Yes, I read the book and saw what they mean but I am not mechanically inclined, my Hubby is not here (he is in the Navy and not home right now) and I am worried that these codes could mean something expensive for me. We do own an OBD, but currently a friend of Hubby's is using it and I do not know their name or phone number to get it back. Any help would be much appreciated!
TY
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Go here http://www.allpar.com/fix/codes.html
Coasty

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Couold be detecting a miss due to vacuum leak. Is the gas cap on completely? That will set a code.

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It's my understanding that you will never have a code repeat. Is that possible 44 instead of 31 & 31? The pause between the numbers and the blinks isn't always uniform.
-Matt

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