Two months ago I pulled code 435 (front O2 sensor slow to respond). I cleared it and a month later it came back. A few weeks after that I pulled code 241 (EGR flow too low) and then a few days later code 436 (rear O2 sensor maximum compensation). It's at the dealer right now and they're telling me that I need new front and rear O2 sensors, a new EGR sensor and a new EGR Valve plus 4 hours of labor = about $1400.

I'm kinda suspicious since they told me that the O2 sensors can take a couple hours if they're rusted on and that seems like it shouldnt take that long. Are they on the level with me about all this, or should i just buy some new O2 sensors and hope the EGR code was just an effect of bad sensors? Any help whatsoever, even explaining to me if the EGR is a big deal at all, would be greatly appreciated.

-1995 850 (non-turbo) 127K miles.