The car starts well.....but when goes down to warmed up idle speed{between 500-1000r.p.m]the motor misses and sputs{youcan here it coming out of the muffler at the back of car.....at driving speeds the motor seems to run fine....but come to a stop sign.....and the rpms drop.....it runs terribly rough...but doesnit stall....I checked the computer codes...Only number 55???ANY IDEAS????what the problem could be???????SINCERE THANKS.....Doug A