1996 Chevy Blazer Troble codes

P0131 Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 1)
P0339 Crankshaft Position Sensor A Circuit Intermittent
P0341 Camshaft Position Sensor Circuit Range
/Performance
P0440 Evaporative Emission Control System Malfunction

Well the first three share a common grounding point on the engine block. I would check that first. Make sure the ground to the chassis and to the battery is good. Also check the connection to the positive feed as well. Make sure that the O2 sensor wires are not shorted somewhere.

The 0440 could be many things. Bad gas cap, rotted out fuel vent line, bad solenoid on the vent line.