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