EVAP System Leak: Evaporative Emission System Leak

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The Evaporative Emissions System, EVAP, with trouble code: P0456, is a closed system that traps fuel tank vapours, such that they are unable to escape into the atmosphere. The only indicator to show you have EVAP System Leak is a glowing check engine light on your dashboard. When this engine light comes on because of a fault in this system, you may not notice the car driving any differently because the car will drive normal.

Evaporative emission system leak is detected, Some factors are responsible for the system trapping fuel tank vapours. Now let’s look at some causes of EVAP system leak.

Causes for Evaporative Emissions System Leak (EVAP System Leak)

  • Loose fuel cap or missing fuel cap
  • Use of wrong fuel cap
  • Leaking fuel tank
  • Evaporative emission system hose leaking
  • Fuel tank leaking
  • Small leak in any of the hoses or fuel tank hoses
  • Small leak in the purge or vent valve
  • Evaporative emission canister

When ‘Check Engine’ Light Comes On

You need to first consult your owner’s manual to establish if it’s actually the check engine light that came on or if it is the service engine light. You may not be able to ascertain if the engine light came on because of an EVAP failure or some other faults unless you try to go through the emissions test lane.

However, you can start by checking the gas cap to ensure it’s tight. Then you may have to consult a mechanic for proper diagnosis. Your mechanic will be able to retrieve the code(s) which will point out the faulty system.

If the fault is an EVAP emissions code, your mechanic would test for leaks in the fuel cap area, EVAP hoses, fuel filler hose, vent valve, EVAP canister, fuel lines, your fuel tank, and fuel lines.

How to Test for EVAP system Leaks (Evaporative Emission System Leak)

There are different codes that are used to identify these leak problems. A repair manual should help you identify the EVAP leak. Evap leaks are quite hard to detect without special equipment.

The various tests for EVAP leaks includes:

  • Vacuum Test 

An engine vacuum gauge can be adjusted to test valves and lines for integrity as all EVAP systems lead to the intake manifold.

First, confirm if the engine idle vacuum is around 21 in. Hg (inches mercury).

Allow the engine to be running while you disconnect the purge valve solenoid electrical connector.

There should be no vacuum in the EVAP system, check to confirm this. The presence of engine vacuum means the purge valve is stuck open.

  • Hand Vacuum Pump

This device can be used to check some EVAP system valves, even when the engine is off.

The purge valve should be closed while the vent valve should be open.

The vacuum gauge will indicate whether the valve is holding pressure. The power valves can be powered manually to check their operation and sealing.

  • Smoke Test 

The smoke test involves blowing smoke into the EVAP system and looking for smoke escaping from a compromised valve, seal, tube, or hose.

This test is probably one of the best ways to test the EVAP system.

  • Bubble Test 

For bubbly test, you can use an air mattress inflator or shop-vac outlet which will not inflate over 2 or 3 PSI.

You can pressurize the system and spray down EVAP system components with a soapy solution.

Where there are leakages, it should appear as bubbles or foam.

EVAP System Leak: How to Repair EVAP Leaks

Fixing an EVAP System Leak is quite difficult. Though the repair can vary in intricacy and cost, depending on the part of the leaking EVAP system you experience

The usual repair procedure simply involves removing and replacing the faulty valves. EVAP system testing and repair is complex.

So it is best to consult a professional mechanic to fix the problem. Ensure the DTCs are reset when the EVAP system repair is over.

Follow this video tutorial on How To Fix Large Evap Leaks On Your Car P0455 if you are experiencing a large evap leak.

EVAP System Leak Fix | How To Fix Large Evap Leaks On Your Car P0455

And it is a small leak, then you have to watch this video.

Evaporative Emission System Incorrect Purge Flow

When the Evaporative control system purge flow is incorrect, it is being identified with the code P0441 (It means the purge flow is higher or lower than it ought to be).

The Evaporative control system (EVAP) stops gasoline vapors that is emitted from the fuel tank to escape into the atmosphere.

What the EVAP system does is to trap and store gasoline vapors in the charcoal canister.

When you drive and certain conditions are met, the fuel vapors are gradually purged from the charcoal canister and burned inside your engine.


  • The Check Engine Light will come on
  • The engine may run a little rough at idle mode.

Causes of Evaporative Emission System Incorrect Purge Flow

  • damaged charcoal canister
  • disconnected purge line
  • vacuum hoses not routed properly
  • bad purge valve
  • vacuum leakages near the purge line
  • faulty EVAP VSV vacuum hoses
  • bad vapor pressure sensor
  • faulty leak detection pump (LDP)
  • rusted filler necks
  • bad NVLD unit (Chrysler)
  • faulty vent valve
  • cracked or clogged EVAP VSV vacuum hoses
  • bad EVAP VSV valve
  • open or short circuit in the purge valve control wiring.


The incorrect purge flow of the evaporative emission system can be diagnosed with a tool and without a tool:

With a scan tool: You can use the scan tool specified by the manufacturer since it has capability to run the test of the whole EVAP system.

The purge valve must be activated using a generic scan tool, and you monitor the EVAP system pressure.

Without a scan-tool: This method involves checking visually for problems without using a tool.

  • check the gas cap to ensure it is properly closed
  • check to confirm there are no cracks or other damage on the charcoal canister.
  • check to see if all vacuum hoses related to the EVAP system are properly connected.

If there are no apparent problems detected, then it means the purge valve needs to be tested next.

The purge valve is usually closed but opens when voltage is applied to its terminals.

You can test the purge valve using a hand-held vacuum pump. It should hold the vacuum when closed with no voltage and open fully with the application of voltage.

Common Causes of Evaporative Emission System Incorrect Purge Flow

A bad purge valve in cars is mostly the common cause for the evaporative system incorrect purge flow.

The purge valve, also called purge solenoid, can be bought from a dealer. It is very easy to replace in most cars.

When a charcoal canister starts malfunctioning, the charcoal pellets are drawn into the purge line and congest the purge valve or the purge lines.

If there are lots of charcoal pellets stuck inside the purge valve, the charcoal canister might need replacement.

Evaporative Emission Control System Vent Control Circuit Malfunction

A fault detected at the vent control part of the evaporative emission control system (EVAP) is identified by the code P0446.

The code P0446 doesn’t openly point to a faulty part, some testing might be required to find the faulty component.

How the EVAP system vent control works

The Evaporative System (EVAP) hinders the vapors of fuel from the fuel tank from escaping into the atmosphere. The fuel vapors get confined in the charcoal canister and this canister is filled with small bits of activated charcoal that absorb and store vapors. As the engine is running, the fuel vapors are purged from the charcoal canister into the engine air intake to be burned. The vent control valve is an electrical solenoid or valve, installed at the back of a car or truck, near the charcoal canister. The vent control valve is controlled by the engine computer (ECM) and it is normally open to control the air supply into the canister. The vent control valve closes when the ECM completes the leak test of the EVAP system. The ECM seals off the EVAP system during the leak test by closing both the vent control valve and the purge control valve.

The ECM monitors the vacuum or pressure in the EVAP system once the EVAP system is sealed. If the EVAP system fails to hold the vacuum or pressure, the ECM identifies an EVAP leak.

What are the Causes of the evaporative emission system vent control circuit malfunction?

  • EVAP system leak
  • Damaged vent control valve
  • bad charcoal canister
  • EVAP system leak
  • faulty EVAP system pressure sensor
  • bad vacuum switching valve (VSV) (Toyota, Lexus)
  • open purge valve
  • jam at the vent control valve, hose, or vent filter
  • open or short circuit in the vent control valve

How to diagnose evaporative emission system vent control circuit malfunction (P0446)

  • Start by scanning your vehicle to confirm P0446 is the only code present.
  • If you discover other codes are present, you need to fix those codes first before commencing with repairing P0446.
  • Check your gas cap to see if it is loose or damaged. Tighten any loose gas cap and clear the code.
  • Examine your gas cap for physical damage and note that damage to the gas cap or its components are not always visible. Replace the gas cap and clear the codes if your gas cap was not loose and if there are no indications of failure.
  • Check for a faulty or disconnected EVAP hoses close to or connected to engine air box
  • Replace cracked and/or disconnected hoses and clear the code
  • Check the fuel tank and charcoal canister for damage and leaks and replace if necessary.
  • Inspect the charcoal canister vent valve and purge valve to check if they are functioning properly.
  • To test, disconnect the valve connector and test with a multimeter to check if the valve is receiving power.
  • If power is absent, then it may point to a wiring problem or a faulty powertrain control module.

Conclusion on EVAP System Leak (Evaporative Emission System Leak)

I hope this post on EVAP System Leak was explicit enough to help you fully grasp the topic. Evaporative Emission Control System prevents fuel vapours from escaping into the atmosphere, which is subsequently absorbed and stored by charcoal pellets in the charcoal canister. It may be considered slightly safe to drive with an EVAP leak, however, you should avoid driving your vehicle for long with a leak. If your Check Engine Light comes on, ensure the gas cap on your vehicle is firmly tightened. If the light is still on, take it to a mechanic for the leak to be fixed.

Rectifying any problem with the EVAP system may be difficult. It is better to take your vehicle to a professional mechanic for proper diagnosis and repair.

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