Error code 12 is a flame sensor error. Buffing it clean with a fiber-tex or similar (soft-pot-scrubbers, not steel wool, not sandpaper) usually does the trick. It's an annual maintenance thing for some installations, goes for years in others. Some modulating-condensing boiler flame sensors have similar issues.
If you don't already have it, download the manual:
You can figure out how to find the flame sensor from the exploded diagram on P.24. (Be sure to turn off the power and the gas before opening the sucker up, eh? ) While you have shut down and open, clean the filter too (an annual maintenance thing in most installations.) Walk yourself through as much of the maintenance protocol on p.16 as you're competent to do before buttoning it all back up.