P0449 – Evaporative emission (EVAP) system, vent valve circuit malfunction


By Reinier (Contact Me)
Last Updated 2016-07-31
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P0449 Evaporative emission (EVAP) system, vent valve circuit malfunction Wiring, EVAP canister purge valve, ECM

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What Does Code P0449 Mean?

OBD II fault code P0449 is defined as “Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction”, and is set when the PCM (Powertrain Control Module) detects that a part of the EVAP (Evaporative Emission Control System) is not working within design parameters. Note that this code refers specifically to the vent valve/solenoid control circuit. Modern EVAP systems typically consist of the fuel tank, fuel filler cap, pressure and flow sensors, a charcoal canister, vent valve with a control solenoid, purge valve with a control solenoid, fuel and vacuum lines, fuel vapor hoses, and electrical wiring / connectors.

The function of the EVAP system is to capture fuel vapors before they can escape into the atmosphere. Fuel vapors are collected and passed through a charcoal canister in which the activated charcoal stores the vapors, before being passed to the inlet tract through a network of hoses to be burned as part of the air/fuel mixture when operational conditions allow this to happen.

The vent valve works in conjunction with the charcoal canister to achieve this, with the function of the vent valve being to allow atmospheric air to enter the EVAP system to help displace fuel vapors as the vapor is purged from the system to be combusted in the engine. On almost all applications, the vent valve is normally open, and the application of a 12-volt circuit current closes the valve when the PCM needs to pressurize the EVAP system to test for leaks. Problems in the control circuit or a failure of the vent valve itself will store a trouble code and trigger the CHECK engine light.

The image below shows the location of a vent valve in relation to the other major components in a typical EVAP system.


What are the common causes of code P0449 ?

The most common cause of code P0449 is failure of the vent valve caused by the presence of dirt or charcoal particles, or a failure of the vent valve control solenoid that prevents the valve from opening or closing. Other possible causes include-

  • Burnt, damaged, shorted, or corroded wiring and /or connectors.
  • Open circuits.
  • Loss of ground.
  • PCM failure is a rare event, and the fault must be sought elsewhere before any controller is replaced.

What are the symptoms of code P0449 ?

In many cases, there will be no symptoms present other than a stored trouble code and an illuminated warning light. However, in cases where the code is the result of the failure of the vent valve to close, a strong odor of fuel may be present. This may, or may not be accompanied by other EVAP system related fault codes.

How do you troubleshoot code P0449 ?

NOTE #1: The vent valve control solenoid forms an integral part of the control circuit, and as such, it must be tested as well.

NOTE #2: Since the purpose of the vent valve is closing off the EVAP system, you will need a vacuum pump with an integrated gauge to diagnose a defective vent valve. You will also need a good quality digital multimeter and a repair manual for the application being worked on to determine the location, routing, function, and color-coding of each wire in the vent valve control circuit.

SPECIAL NOTE: If other codes are present along with P0449, follow the rule that says all codes must be diagnosed and resolved in the order in which they were stored. Failure to follow this rule could result in a misdiagnosis. In some cases, most notably on some Nissan and Infinity models, a failed vent valve will trigger code P0455, while some GM models will display code P0466. Both codes are caused by dirt entering the vent valve through a broken screen on the vent valve inlet side. Also note that on these models a defective vent valve can make it almost impossible to fill the fuel tank, since the air in the tank cannot be vented.

Step 1

Record all codes present, as well as all available freeze frame data. This information can useful to diagnose intermittent faults.

Step 2

Perform a thorough visual inspection of all associated wiring and connectors. Look for shorted, burnt, damaged, broken, or corroded wiring and connectors. Repair wiring and/or connectors as required. Clear all codes when repairs are completed, and rescan the system to see if the code returns.

Step 3

If no visible damage to wiring is found, perform resistance, ground, and continuity checks on all associated wiring, and compare obtained readings with the values stated in the manual. Pay particular attention to the voltage that reaches the solenoid. Most solenoids require full battery voltage to work, so be sure to check that a low battery charge is not the cause of the problem.

Repair or replace all defective wiring to ensure that obtained readings fall within specifications. Note that the vent valve control circuit must be disconnected from the PCM during resistance and continuity checks to prevent damage to the controller.

Step 4

If all electrical values fall within specifications, it should be possible to manually activate the vent valve repeatedly with the scanner if the scanner has that capability. Activating the vent valve solenoid repeatedly will often reveal intermittent faults, which can sometimes be extremely difficult to trace and repair. In extreme cases it may be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

Step 5

If the scanner does not (or cannot) activate the vent valve solenoid, consult the manual on the location of the valve. On some applications the valve is connected to the charcoal canister, while on others, the actual valve may be located some distance away.

Disconnect the valve solenoid from the system, and follow the directions in the manual on the correct procedure to apply battery voltage to the solenoid directly. If the control solenoid is fully functional, there will be an audible click as soon as current is applied. If there is no “click”, the solenoid may be defective, or the solenoid shuttle may simply be stuck. Test the solenoid’s internal resistance, and if it agrees with the value stated in the manual, prepare to remove the solenoid for further testing or replacement.

Step 6

Once the valve and solenoid is removed from the vehicle, attach the vacuum pump to one opening. Note that since vent valve solenoids are normally open, you need to apply battery current to the solenoid so that the valve closes. Make sure that the solenoid is properly grounded during this step.

Draw a vacuum with the pump until the gauge registers a vacuum value within the valve’s operating range. Assuming that the test equipment is not defective in any way, the vacuum will hold steady if the valve closes fully. A decaying vacuum indicates that the valve is leaking, or not closing fully. Replace the valve if it does not hold a vacuum.

NOTE: Note that during normal operation the vent valve contains a pre-defined pressure, and not vacuum. However, since it is easier to test the valve with a vacuum as opposed to a positive pressure, it is important not to exceed the vent valve’s design parameters.

Doing so could result in a false positive result, meaning that a perfectly good valve could be condemned simply because it is expected to perform outside of its design parameters. Use this handy converter to convert the EVAP system’s design pressure to a usable vacuum for testing the vent valve.

Step 7

When replacing, or re-installing the vent valve after testing, it is always a good idea to inspect all associated vacuum lines for signs of damage, cracking, splitting, or other issues. Although vacuum leaks usually trigger codes other than P0449, making sure that all vacuum lines are in good shape is a common-sense preventative measure that could prevent issues and problems later on.

Once the vent valve is replaced or re-installed and all electrical connections are secure, clear all codes and retest the system to see if the code returns. If it does not return after several drive cycles, the repair can be considered as having been successful.


Codes Related to P0449

  • P0446 – Relates to “Evaporative Emission Control System Vent Control Circuit Malfunction.”
  • P0447 – Relates to “Evaporative Emission Control System Vent Control Circuit Open.”
  • P0448 – Relates to “Evaporative Emission Control System Vent Control Circuit Shorted.”

BAT Team Discussions for P0449

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