2002 Mercury Cougar 6 cyl sporadically idles rough and stalls at times. Check engine light illuminated. Home computer diagnostic displayed code PO171. Red led light which claims it involves more than one system. Some of the lights on the diagnostic began to flash. EGR, oxygen sensor, and heater oxyygen sensor. Car seems to idle worse when heater is on. Replaced EGR valve two years ago. Any suggestions would be appreciated. Would rather not spend $600 dollars for labor.