Here is what trouble codes on your Dodge Neon mean:
a. P0442 EVAP emission control system leak (small). Could be something as simple as a loose or leaking gas cap.
b. P0455 EVAP emission control system leak (large). More than likely a broken or disconnected EVAP hose.
The EVAP system on your Dodge Neon is not a very complicated system. The fix here requires tracing all the EVAP vacuum hoses from the engine to the charcoal canister to the gas tank, including inspecting the vent solenoids and roll-over valve (located on top of the gas tank).
1. Start by inspecting all the evap hoses, as mentioned above. Look for breaks, cracks, and disconnected lines.
2. An inexpensive attempt to correct P0442 (after P0455 has been fixed) is to replace the gas cap and drive your Dodge Neon for 100 miles over the course of a week. Hopefully the small EVAP leak fault was due to the gas cap and a new cap will fix the problem and turn off the check engine light.
3. Next you should check for the proper operation of the EVAP purge solenoid. The EVAP purge solenoid will be located near the charcoal canister and allows for gas tank fumes to enter the intake manifold when activated.
4. Finally, if no faults have be found with any of the evap hoses and/or solenoids, you should inspect the EVAP canister itself. Look for cracks, as well as hoses leading to it which might be loose or disconnected.
Once a fault is found a repaired you should reset your Dodge's engine control computer (ECC), clearing the trouble codes. Then drive your Dodge Neon through a drive cycle (basically drive the car for at least one week in various driving conditions) in order to prepare it for the smog check. Just ensure the check engine light has turned off and the trouble codes have been cleared. The ECC can be reset using a scan tool or by disconnecting the battery for 10 minutes.