Known as the "three-quarter small-block" among Chevy enthusiasts, the 4.3-liter was one of the more interesting factory modifications of the old-school 350 engine. While popular at its introduction and through its production run, this engine, like all others, got a few updates after 1996 -- and not all of them have proven so popular.
Starting in 1996, the 4.3-liter -- like all other engines sold in America -- got a massive computer overhaul in the form of OBD-II. One major difference between OBD-II and former systems was that the new system monitored something known as "evaporative emissions." To monitor evaporative emissions -- fuel evaporating out of the tank -- these systems relied on pressure sensors in the fuel tank vent tube. Engine vacuum, pulling through the charcoal canister and the fuel tank vent tube, kept the tank at a constant negative pressure, keeping vapors contained inside instead of leaking out through the vent or gas cap.
Code P0456 translates as an "EVAP System Pressure Sensor Low Input," which is another way of saying that the voltage coming from the sensor is too low. This is a self-diagnostic code indicating that there's a fault with the pressure sensor or the sensor circuit. You may also get code P0440 with it; this would normally mean a vacuum loss in the system, but in this case is probably the result of a bad sensor. This fault won't affect driveability at all, since the engine control computer doesn't care what happens with the EVAP system. But it you should deal with it, if for no other reason than that the check engine light will stay on till you do. An always-on check engine light is as bad as no check engine light, so you won't know if something serious goes wrong later.
The fix could be as simple as making sure your gas cap is on tight.
