Troubleshooting EVAP System Leak DTC Codes

Troubleshooting P0 Codes can be tricky, especially if it involves the EVAP System because it can be tough to find where the problems are happening. That's why we are here to help you learn how to troubleshoot and diagnose your car's mechanical problems. 


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 need more specific advice, here are articles on individual P0 Codes:

  • P0441 - Evaporative Emission System Incorrect Purge Flow
  • P0442 - Evaporative Emission Control System Leak Detected (Small Leak).
  • P0455 - System Gross Leak Evaporative Emission
  • P0456 - Evaporative Emissions System - Small Leak Detected
  • P0457 - Evaporative Emission System Leak Detected (Fuel Cap Loose/Off)


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. Here are the specifics diagnostic steps to figure out what is causing the P0 code.

Step 1: Check the Gas Cap and Filler Tube

The first thing to do is to check your gas cap. If the gas cap is not tightly secured, tighten it and drive for a few days to see if the check engine light goes off. If the light goes off, then the gas cap was the issue. If not, then it’s time to move on to the next step.

Step 2: Visually check Hoses

Check hoses under the hood linked to the EVAP system to make sure there are no abnormalities or cracks. Next lift the car and check hoses linked to the canister and fuel tank check for cracks or abnormalities. If nothing visual can be seen, move to the next step.  

Step 3: Check the Purge Valve and Vent Valve

Next is to check the purge valve. The purge valve is responsible for regulating the flow of fuel vapors from the charcoal canister to the engine using vacuum. If the valve is faulty, it can cause an EVAP code. 

To check the Purge Valve, follow these steps:

  1. Locate the purge valve. It’s usually located near the engine, on the top or side of the intake manifold or throttle body. Some models might be located near the canister*
  2. Disconnect the electrical connector from the valve.
  3. Remove hose that applies vacuum to the valve
  4. Using a vacuum pump, apply vacuum to the valve from where you disconnected the hose. 
  5. If the valve does not hold vacuum, it’s faulty and needs to be replaced.

To Check the Vent Valve follow these steps:

  1. Disconnect the electrical connector of the valve.
  2. Disconnect the hose that links the Valve to the system
  3. Using a Vacuum pump apply vacuum to outlet port of the vent valve
  4. With the Valve energized the vacuum should sustain. If not vacuum is sustain it is a faulty Vent Valve

Step 4: Check for Vacuum Leaks

If both the gas cap and purge valve are fine, the next step is to check for vacuum leaks in the EVAP system. A vacuum leak can cause an incorrect purge flow, which triggers the P0441 code. 

To check for vacuum leaks, follow these steps:

  1. Start the engine and let it idle.
  2. Use a smoke machine to inject smoke into the EVAP system. If you don’t have a smoke machine, you can use a cigar or incense stick to create smoke. For more information about smoke testing, here is an article about How to Smoke Test the EVAP System to Diagnose a Leak.
  3. Inspect the EVAP system, including the vacuum lines, charcoal canister, and purge valve, for smoke leaks. If you see smoke coming out, you’ve found the leak.

Step 5: Replace Faulty Components

If you’ve identified a faulty component, such as the purge valve or a vacuum line, replace it. Make sure to use OEM parts or high-quality aftermarket parts from a reputable brand.

Step 6: Clear the Code and Test Drive

After fixing the issue, use an OBD-II scanner to clear the P0 code. This will turn off the check engine light. If the code comes back, it means there’s still an issue with the EVAP system, and you need to repeat the steps to diagnose and fix the problem.

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