What can be done about 5 cylinder misfiring

I have a 1999 GMC Safari. Had it at the mechanics and he replaced lots of stuff. The check engine light was on. He said that the number 5 cylinder was misfiring and only the chevy dealer could tell me what was wrong. It had check engine code po300, po175, p1870, po302, po305. Do you what might be wrong with it and is it even worth fixing