Evaluating EVAP Systems & EVAP Leaks


In this article you will learn everything from what an EVAP system is and why you need one to common issues and codes associated with the EVAP System. In addition, you will learn how to diagnose and find EVAP leaks and finally what parts you might need to fix an EVAP leak once you find one.

Use the table of contents below to jump to a part of the article that suites you best.


  1. What is an EVAP System and Why Do I Need One
  2. Diagnostic Trouble Codes 
  3. I Have a PO4xx Code What Do I Do
  4. Smoke Test to Find EVAP Leaks
  5. Smoke Test for EVAP Leaks
  6. What to Do After Finding Leak
  7. Wrapping Up the Job

Understanding the EVAP System & Learning How to Diagnose EVAP Leaks


Gasoline (Fuel) vapors are dangerous, but maybe not for the reason you think. They can react  with sunlight to cause photochemical smog, which is dangerous to breathe. Fuel vapors come from two sources: Refueling the car, and fuel evaporation to air through a leak. 

Onboard Refueling Vapor Recovery (ORVR) Function

When the vehicle is refueled, the air and fuel vapor mix that is in the tank must go somewhere. Since we can’t release the vapors to the air, we pass the air/vapors through the special “activated” charcoal in the canister, which adsorbs the fuel vapors and stores them. The cleaned air is exhausted through the canister vent valve. Once the engine is started and the Powertrain Control Module (PCM) computer is satisfied that it is okay to do so, the computer allows the vapors to be slowly cleaned off from the charcoal and burned by the engine. This cleaning prepares the canister for the next refueling event. 

Evaporative Emissions System (EVAP) Function

Automakers are required to design cars so that the fuel system is sealed and does not allow fuel to evaporate to the atmosphere. The PCM conducts vacuum or pressure tests on the fuel vapor and tank system, looking for the presence of leaks, either large (gross) or small, that could allow vapors to escape to the air. The strategies for this vary from car to car, but most involve sealing the fuel system and watching the system air pressure to change over time. 



If a leak is detected, the PCM may set a Diagnostic Trouble Code (DTC). These DTCs generally are 5-digits in this format: P04xx. The P means Powertrain computer, where the DTC is stored, the 0 means it is a generic code for all carmakers, and the 4 means EVAP system. The last two digits define the type of problem: low flow, electrical problem, small leak, large leak. You can retrieve these DTCs with an inexpensive code reader. The driver will notice that there is a problem because the amber “Check Engine” or “Service Engine Soon” MIL (Malfunction Indicator Lamp) is illuminated. 

If you are not familiar with Diagnostic Trouble Codes, you will want to brush up on this by visiting our article, “What is OBD-II.”



You have entered the trunk of a tree with many branches. The main branches each correlate to a DTC. Here is a note from 40 years in the diagnosis business: Most problems are simple ones that are located with a thorough visual inspection. Carefully check the fuel filler neck for cracks or rust, vacuum lines for leaks or contamination.

For example, a P0440 is the most commonly occurring DTC. It indicates, in most cases (Check your vehicle’s service information!) that a gross leak exists in the fuel system. The most common cause of this DTC is a loose or missing fuel cap. Replace the cap if the gaskets or O-rings show signs of cracking or wear. If the fuel cap has already been replaced, you can move to the next step, which is diagnosis and testing of components.



The purge solenoid is the “front door” to the system. It allows vapors to be burned by the engine. If it fails to open, the system can’t empty the charcoal from its adsorbed hydrocarbons. If it fails to close, it causes the system to be drawn to low pressure (vacuum) at the wrong times.

Testing the Purge Solenoid

Testing the Purge Solenoid is fairly easy if you have an advanced scan tool. But what if you don’t have an extra $1500 to invest in a scan tool? This component can be tested without the scan tool. First, locate the solenoid valve. It has two or three wires attached to it, and a vapor line that goes toward the fuel tank. The other end of the solenoid will either be screwed into the intake manifold or connected to the manifold with a hose. 

Disconnect the vapor line that goes toward the fuel tank. Attach a vacuum pump with gauge to the purge solenoid inlet fitting and apply vacuum with the pump. The vacuum should remain steady when the engine is not running.

If the solenoid fails this test, you can confirm the results visually by removing the solenoid valve and applying smoke under light pressure to the inlet side of the solenoid. If it is leaking, smoke will be visible on the outlet side of the solenoid valve. Replace the solenoid valve, clear DTCs, and retest the system.



If you got this far, your DTC has indicated that there is a leak in the system, and you have already tried replacing the fuel cap and clearing the DTCs, but the DTCs came back within a few days. It’s time to locate the leak.

The general process to leak location is to seal the system completely, then insert smoke under a slight pressure and observe for leaks. 



Solution #1

The first way to close the vent valve is to use an “advanced” scan tool that has capability of what is called “bi-directional” or two-way control. This means that code readers won’t help.

Most Code Readers are just that – they can read information that the PCM sends out. In order to get a scan tool that has the ability to command the car to activate things, you will have to spend a little cash. Check the links at the end of this article for our suggestions for tools including scanners. You can spend $500 to $5000 for these tools.

Solution #2

The second way to close the vent valve is to use a jumper wire to activate the solenoid. You can do this at the PCM connector, but I have to caution you that this is risky, so if you aren’t sure of yourself, this is the time to throw in the towel.

If it is accessible, you can also jump power and ground right at the vent solenoid. You will have to determine if the solenoid is power, or ground side controlled. The Vent Valve is generally off (open) when the engine is off. The PCM closes this valve only during the automated test process. Be sure to use a fused jumper wire when testing.

Whichever method you choose to seal the system, if you don’t close the vent valve before you apply smoke to the system, you will just fill the back of the car area with smoke and you won’t find the problem.



Recommended Tools:

  1. Shop Series Automotive Smoke Machine
  2. Your choice of one of the tools below to access the EVAP System.
    1. EVAP Service Port Adapter
    2. Gas Cap Adapter (Quickest & Easiest Method)

This is another step that sounds so easy until you try it. We recommend testing indoors in good light where there is not a breeze. Get a good, very strong flashlight. Using a laser pointer can help detect tiny amounts of smoke. Make sure that the smoke machine does not run out of smoke during the testing process. Always use a smoke machine that limits the applied pressure! Our AutoLine Pro Shop Series smoke machine is a powerful, yet budget friendly solution recommended for both veteran and DIY mechanics alike. Whichever smoke machine you decide to use, you will more than likely need to pick up an EVAP Service Port Adapter or Gas Cap Adapter to properly access the and smoke test the EVAP system.


Locate the access port for the EVAP system under the hood of the car. The port is covered by a green plastic cap which simply unscrews. There is a valve core under the cap, but the smoke machine should be able to push smoke right past it.

If the vehicle does not have a test port, you can connect to the tank via the fuel filler using the appropriate adapter. Again, fill with smoke and check for leaks.


EVAP systems are designed to operate and test at a very low pressure of about 12 inches of water – that is about one-half PSI. Testing them at pressures above 7 psi could blow off hoses or damage the pressure sensor. Always use a smoke machine that regulates that applied pressure to less than 7 psi.



So, what’s leaking? If you are having a hard time pinpointing the leak, you may have to remove the rear seat or even drop down the gas tank for closer examination.

Don’t forget to replace all parts that are questionable. We’ve seen rust on the retaining ring of the fuel pump cause the pump housing to crack and create a leak.


You have several choices for EVAP system parts. Consider the amount of work to replace a defective part before you make a purchase decision based on price alone.

“The Dealer” is the source that generally offers the highest quality (OE), highest price replacement parts. Many aftermarket parts perform or even outperform OE parts for a lower price. However, some part makers sacrifice quality for price, and you will end up paying the cost. Personally, my rule of thumb is that if it takes more than two hours to change the part, I am buying it from the dealer or a reliable parts source such as Rock Auto (not a sponsor).

Easy access parts can be located by searching for the lowest cost on sites such as eBay motors. Local parts vendors such as AutoZone, Parts Plus, NAPA, and O’Reilly often have quality parts at competitive prices. 



Clear DTCs and Retest

After repairing the system, clear the DTCs using your scan tool or code reader. You can also clear codes by disconnecting the negative battery terminal for ten seconds, but you may have to reset clocks, radio stations, and other presets if you do it this way. Either way should turn the MIL off. 

Whichever way you clear the codes, the computer will have to run its self-tests (called Monitors) before it is happy that the system is working properly. This can take a few weeks of driving for some cars.

If you would like to learn more about Monitors, OBD-II, and Diagnostic Trouble Codes, visit our article titled, “What are DTCs and Codes.”