4 th cylinder miss fireing
after a trip cross country wa state to michigan and an uneventfull trip, got great gas millage and my baby ran so good had a trip check done and new breaks...before the trip...destination arrival , into a month or so the care is lagging on small up hills..feels like it skips when giving it gas, my baby has 150.000 miles and my old mechanic said she was still in good shape..put new heads on a few years ago..now this...was told 4th cylinder was missfireing...is it expensive and what causes it..thanks guys..just a ole gal wanting to keep her baby in good running order..any advice is appreciated..oh and the check engine light stays on..:(