What does it mean?
EVAP System Basics Fuel volatility is an aspect of fuel with advantages and disadvantages. On the plus side, the better a fuel vaporizes the better it burns, which is why we burn gasoline and diesel in our vehicles instead of, say, wood or coal (though these can be used with sufficient processing). On the negative side, however, fuel vapors have been found to be harmful to the atmosphere and human health, so controlling those vapors is particularly important.
The evaporative emissions control system, typically referred to simply as the “EVAP” system, has been designed to prevent fuel vapors from escaping into the atmosphere. The EVAP system is essentially a network of tubing and valves designed to store and release fuel vapors at the right time without permitting them to get into the atmosphere. The EVAP system temporarily stores fuel vapors in an activated-charcoal canister until they can be burned.
Under certain engine operating conditions, the EVAP system will open to the engine intake, and fuel vapors will be “purged” from the canister burn in the engine. The ECM (engine control module) uses pressure and voltage sensing and logical programming to control the EVAP system and to verify that it is sealing and venting when and where it should.
In the case of the purge cycle, the ECM opens the purge valve and the vent control valve to allow engine intake vacuum to pull fuel vapors into the engine to be burned. The vent valve needs to be open to allow fresh air to flush out the charcoal canister, “cleaning” it of stored fuel vapors and making it ready for another charge. Part of the EVAP self-test includes voltage and resistance sensing.
Some ECMs may also use logic sensing to determine if the vent control valve is working properly – if the vent control valve is closed, then opening the purge valve will generate a vacuum, for example. If the ECM attempts to activate the vent control solenoid but detects incorrect voltage feedback or incorrect vacuum or pressure feedback, it will lock out EVAP function, set a diagnostic trouble code (DTC), and illuminate the malfunction indicator lamp (MIL).
DTC P0448 is defined as “Evaporative Emission (EVAP) Vent Control Short Circuit.”