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Diagnostic Trouble Codes (DTCs) are never fun to look up, but unfortunately, your vehicle sometimes leaves you no other choice. If your Check Engine Light is on and the OBD-II scanner reads P0456 code, it’s good to familiarize yourself with some basic facts about this DTC, as well as ways you or your mechanic can diagnose and repair it.

The Meaning and Cause of P0456 Code

To start, let’s cover the basics: there are two main DTCs, generic and manufacturer-specific. Generic codes, also known as SAE, apply to all vehicles equipped with the OBD-II and are pretty easy to find online. Manufacturer-specific codes, on the other hand, are harder to find online as not all manufacturers are willing to share their information. Thankfully, the P0456 code is a generic powertrain code, meaning it applies to all OBD-II equipped vehicles. However, even with the generic codes, specific diagnostic and repair steps may vary from model to model.

But What Does the P0456 Code Mean?

To put it simply and bluntly, this is a code for “evaporative emissions system – small leak detected”. In other words, when the scan tool reads the P0456 code, your OBD-II system has detected a lack of pressure at the Fuel Tank Pressure (FTP for short) sensor within the Evaporative Emission Control System (EVAP for short), indicating a small leak. By the way, your Check Engine Light won’t turn on if a leak is detected only once – the test for this leak must fail twice in a row for that light to start shining.

What Causes the P0456 Code?

As usual, things are never black or white with the DTCs, meaning there can be quite a few causes for the code. However, this particular code is easier to understand than, say, the P0303 code is. This is because the P0456 code is detected in a very specific way. So, to understand the possible causes, you first need to understand the process behind detecting the P0456 code. Luckily, we made this process super-simple for you:

  • The Powertrain Control Module, or PCM for short, signals the purge control valve to close the system and start the evaporative vacuum pump so that the system can be brought into a state of vacuum. Why is this done, you ask? So that the PCM can test the system for leaks.
  • If the system detects a loss of pressure in the vacuum during the test, the PCM stores the code and waits until a second test is done. Remember how we said that your Check Engine Light won’t start shining until the leak test fails twice in a row? So now, if the second test detects the leak again, the engine control module will store the P0456 code and activate the Check Engine Light.

Because the leak typically occurs within the system components or hose connections, the most common causes are:

  • The purge vent valve and/or gas cap vent not sealing properly
  • Leaking or disconnected EVAP hose
  • Leaking fuel tank
  • Damaged canister vent valve
  • Leaking charcoal canister

The Symptoms of the P0456 Code

Because the EVAP is a closed system, the drivers usually don’t notice any symptoms besides their Check Engine Light shining. This occurs because the EVAP doesn’t control engine management, only the fuel tank vapors.

Saying that, here is what most drivers will notice:

  • Check Engine Light is on (always occurs)
  • Fuel smell (not that common)
  • Decreased fuel economy (not that common)

Diagnosing and Repairing the P0456 code   

To properly diagnose a DTC, a thorough inspection of the vehicle is necessary. While you can do some of the diagnostic steps yourself, it’s recommended you take your car to a professional, unless you’re one. This is advisable because mistakes are common with the P0456 code. Often, EVAP components are replaced in error when a vacuum leak is present or the fuel cap is not properly tightened.

In any case, a mechanic will do the following:

  • Connect the DTC scan tool or reader to the connector. Note all freeze-frame data because this can reveal when the fault occurred (if you want to, you can do this step yourself; just make sure you have a proper DTC scanner).
  • Thoroughly inspect the complete vapor purge valve system. This includes the hoses and connectors as well as the purge valve itself. Look for loose or damaged connections and if anything is loose, tighten it, if anything is damaged, replace it.
  • Check the purge valve for any blockages, such as dirt or debris that could prevent a complete seal of the vapors.
  • Inspect the fuel tank and charcoal canister for any leaks or damages and replace whatever needs replacing.
  • Check the purge valve and charcoal canister vent valve for proper operation. This is important because these valves often cause leaks due to minor damages/blockages. To test them, a mechanic will disconnect the valve connector and see if the valve is receiving power with a multimeter.
  • Use a smoke machine to perform a smoke leakage test on the fuel vapor system. Also use the test vapor port to determine the location of the vapor leak.
  • Finally, retest the system after repairs/replacements are completed. If nothing needed repairing, connect the DTC scanner and note all stored codes as well as freeze frame data as this can be useful in diagnosing intermittent conditions.
  • Clear the codes and perform the test drive to see if the code returns.

Keep in mind that a thorough inspection is an absolute must if you want to avoid misdiagnoses. Often, people will assume that the purge control valve is damaged and replace it, without actually performing a proper inspection of the entire system. And while the purge control valve may indeed be the problem, sometimes something as simple as broken wiring may be causing the P0456 code.

To sum up, these are the most common repairs for the P0456 code:

  • Repairing/replacing the leaking gas cap
  • Repairing/replacing the leaking purge valve
  • Unclogging/replacing the blocked/broken purge vent valve
  • Replacing the leaking hoses or connectors within the system

Additional Information About the P0456 Code

The P0456 is not a common DTC for most vehicles, however, those that come with the Check Engine Light on and the OBD-II scanner that reads P0456 code, usually need a replacement of either the fuel cap, purge control valve or the vent valve, as at least one of these things is no longer able to hold in the vapors, resulting in the OBD-II detecting leaks.

Often, the leak(s) causing the P0456 code is too small to see, which is why a smoke test is usually necessary. For this reason, it’s advisable you let an expert thoroughly inspect your vehicle before diagnosing and repairing it.

Code Explanations

These links will provide more detailed insights into what code means in your vehicle:

P0303 Code
P0155 Code
P0430 Code

Be sure to also read our essential guide to the best OBD2 Scanner.

Sources:

  1. How to Read and Understand OBD Codes – wikiHow
  2. P0456 OBD-II Trouble Code – YourMechanic

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