Crankshaft sensor replaced, but engine light is still on and tachometer reads "0". What else could be wrong?

For a couple months, my car had been having problems with the tachometer******** irratically up and down while driving and the check engine light was on. Also, when I would turn a corner, it would buck a lot and almost stall out. Then, a week ago, when I was driving to work, my car just died (with no warning). After it sat for a little bit, I was able to get it started somehow. I took it to my mechanic and he told me that the crankshaft sensor needed to be replaced because it was sending error codes through the wires. He replaced the sensor at a cost of $420, but my check engine light is still on and my tachometer still reads "0" at all times. Also, when I'm stopped at a light or stop sign, my car bucks and it feels like it's going to stall. It doesn't, I'm afraid it's going to any time. Does anyone know what could be causing the engine light to stay on and the tachometer to stay at "0"? Any help would be greatly appreciated!