my bible says that code 12 = low battery voltage. The air bag diagnostic monitor measures the voltage at pin 14 of the air bag diagnostic monitor connector. The voltage at pin 14 should be equal to battery voltage. If the voltage at pin 14 drops to less then 9 volts the air bag diagnostic monitor will store in memory and flash out on the air bag indicator a DTC 12. Should the loss of battery voltage at pin 14 be intermittent or repaired the air bag diagnostic monitor will flash out a DTC52 on the next ignition switch. Check your battery, check your connections. By check I mean remove them, clean them and then tighten them up till you can move them. You just checked your connections....
there is a ground connection right next to the battery on the core support. The wire from the battery is maybe a foot long before it terminates there. Remove/scand/scrap/clean and then retighten. Other places to check are the power lug on the alternator but you really should just have to check that the wire is tight and will not move. And really before removing anything if you have a meter check your voltage with the key off, and with the car running and let me know what it says. How old is the battery/alt? Under drive pullies by chance?
I would probably start with the meter myself honestly. You can pick up cheap ones at autoparts stores or harbor freight for $10-20 that will do the basics which is all you need and every tool box should have a meter.
ok so the battery is good. Now remove/clean/tighten the connections down and see if it clears it out. You can always do a quick test and grab the battery connections and try to see if you can turn them. If they move at all, it needs cleaning and then tightened down so it will not move.
its amazing how just a slightly loose cable can cause problems. Did you just tighten them down or did you remove/clean and then tighten them down? If you didn't clean them it sounds like there might be a slight layer of corrosion or resistance that has built up and if so you could easily gain .5v to a full volt before/after.
good man people really just do not understand how a small/thin layer of corrosion can stop/restrict electron flow. Most people just think that its all good when metal touches. I learned all this myself as well the hard way. I didn't tighten down a battery cable enough and you could twist it 20-30 degrees. Every 2-3 months the car would not start and I would pop the hood, twist the battery cable back/forth to clean off the oxidation and then it would start. Later I went and tightened it down, but it still have a 5 degree movement but I thought that would not matter. Now it tood 5-6 months till it quit and I had to do the same.