need help with codes 12 &45 on chevy motor?

a code 12 is the first code you always get on a diagnostic, it means system normal (ecm o.k.)

and the code 45 is o2 sensor or circuit, rich exhaust detected (driver's side on 2 snsr sys.)

rarely do o2 sensors fail so more than likely your problem lies elsewere. there's probably a vaccum leak some where, i.e. egr gasket, intake manifold, etc. there may be a vaccum hose that's loose or cracked that needs to be replaced.

not long ago I worked on an impala w/ similar problem and it turned out to be the MAP sensor! something is making your car run a little richer than normal

Hope this helps.

12 is the default code, it flashes 3 times at the beginning and 3 times at the end of diagnostic sequence..

45 is Rich exhaust indication, sensor voltage high after one minute of engine run time.

Consult repair manual for fix. I use Haynes manual

U.P.