Error code P0446 is one of the many DTC or diagnostic trouble codes that apply to certain vehicles. This code indicates the possibility of a malfunction in the vent control circuit of the evaporative emission control system (EVAP). This code is set when the vehicle’s engine computer recognizes that there is a fault at the vent control of the EVAP. P0446 does not, however, point to a direct fault or defective part. Moreover, description of the fault may vary depending on the car manufacturer.

The P0446 DTC is only an informational code which means that it does not essentially indicate a direct defect but a general fault. It is a common diagnostic problem to several vehicles and because it is an informational code, only the general information is likely included in this article.


The PCM or powertrain control module works in monitoring the EVAP (evaporative emission)’s performance through application of a prearranged level of vacuum and then at the same time, it monitors the decay rate of the vacuum. On the other hand, the evaporative control system is what captures any kind of raw fuel that evaporates coming from the storage system – fuel cap, fuel tank, etc. As dictated by accurate operating conditions caused by the engine temperature, load and speed, the EVAP works on storing and purging the fuel vapors that were captured and get them back to the combustion process.

A vent valve is utilized by the carbon canister, which stores the fuel vapors in order to ensure that only fresh air comes inside of the EVAP system. The very purpose of the vent valve is to close and seal the vent in order for the EVAP system to be able to pressurize which ensures there will not be any leaks. Because of the valve, the fuel vapor is displaced while it is being purged through the combustion process.

Following this process, it is the error code pp0446 that indicates if when there is an inappropriate drop of voltage in the vent valve circuit in the EVAP system. This drop can be caused by too much or too little electrical resistance. When this happens, a breakdown is about to happen in the vent valve of the EVAP system.

Common Symptoms

The common symptoms indicating P0446 include the obvious symptom of illuminated malfunction indicator lamp (MIL).  Apart from that, there may be a conspicuous fuel odor that is mostly caused by a release of fuel vapor. On the other hand, there are cases wherein drivers do not see any adverse conditions that may indicate there is any problem.

Possible Causes

There are a number of possible reasons that could have caused the error code P0446. Such causes may be one or a combination of the following:

  • Faulty vent valve
  • Defective PCM
  • Blocked vent valve
  • Excessive, open or short resistance on the control circuit in the vent valve
  • Defective charcoal canister
  • Leak on the EVAP system
  • Blocked vent hose or vent filter
  • Defective pressure sensor in the EVAP system
  • Defective VSV or vacuum switching valve in Toyota and Lexus vehicles
  • Stock open solenoid or purge valve

How to Check

It is said that there are PCMs that might trigger P0446 in the case of the correct level of vacuum has not been reached while in leak test. On the other hand, here are some of the common tests when diagnosing for evaporative system faults:

  • Use the code and freeze frame information as baseline for verifying any repair
  • Perform close visual inspection of any and all accessible components of the EVAP system to see for signs of degradation or physical damage.
  • Use a scan tool for inspection and at the same time paying close attention to the readings on the Fuel Tank Pressure.
  • Verify if the vent valve and purge valve is working properly and are capable of holding vacuum for a continued amount of time.
  • Test whether the fuel cap fits perfectly onto the Fuel Tank Filler neck.
  • In case all of the components of the evaporative system seems to be working properly, then performing a smoke test for the whole EVAP system is a must.

How to Fix

The P0446 DTC is an informational code which means that it does not indicate specific or direct fault in the system. It simply highlights that there might be a problem in the EVAP system which needs to be fixed. Furthermore, error code P0446 is described differently for each vehicle make. However, if you have P0446 trouble code, here are few things which you can try to fix it:

  • Repair the short, open or resistant control circuit
  • Replace your vehicle’s PCM
  • Replace the EVAP system’s vent valve

Most important, in order to fix the problem, is to perform the tests above so that the real problem will be identified. Once the direct problem has been identified, apply the appropriate solution for the issue.

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