EGR sensor bad?
My 2002 Sable has been running like crap for three weeks, starting when the engine light came on. The car sort of lunges or surges periodically while driving, particularly at 25-45 mph. It is as if the engine loses power and then regains it very rapidly, sometimes over and over again but with no regular interval, and with no distinctive sound. My gas mileage has also been awful. My friend suggested that it could be the EGR sensor, and lo and behold when we plugged in his diagnostic tool that is what it read out. Here are the error codes.
p1400 differential pressure feedback egr circuit low input
p0402 exhaust gas recirculation flow excessive detected
p0401 exhaust gas recirculation flow insufficient detected
What does all this mean? Any help is greatly appreciated.