P0496 – Is Your EVAP System Not Purging Right?

In most cars today, there is an abundance of systems and fitments designed to minimize fuel usage as much as possible. Higher MPGs is an easy win-win for your wallet and the environment. Hence, why it’s no wonder that so many cars feature clever tech like EVAP control, ensuring that not even a single fuel molecule is wasted away. Alas, it only works until a P0496 error code appears.

Your car’s EVAP (or ‘evaporative emissions control’ system) is a key inclusion in many newer vehicles. Its role is quite simple – creating an airtight vacuum seal around the gas tank. Thus, making sure fuel doesn’t leak out into the atmosphere, which is a major cause of air pollution. Plus, preventing diesel or gas from being wasted, as the EVAP keeps them sealed inside the gas tank, ready to be burned.

For the most part, and it’s been this way since the 1970s, EVAP systems are fairly reliable and don’t require a lot of maintenance. But unlucky you, EVAP controls are still susceptible to fuel vapor leaks, which can prompt a P0496 error code to appear. While it might seem trivial, fuel leakage isn’t just a serious safety hazard, but it could put a serious dent in your wallet, as well as a car’s driveability.

• What Is EVAP?
• Meaning
• Causes
• Symptoms
• Diagnosis
• Final Thoughts

What Is Your Car’s EVAP System, Anyway?

Before we get into what a P0496 error code means, it might help to provide some context as to what an EVAP system does. So, how does an ‘evaporative emissions control‘ device work on your car? Fun fact; automobile emissions not only come from the tailpipes following combustion. Alternatively, it could also emanate from your fuel system, through what’s known as “evaporative emissions”.

This is the case, as fuel is an extremely volatile compound and could evaporate at room temperature. In doing so, it releases extremely harmful organic pollutants into the atmosphere. It’s applicable for gasoline more so than diesel, as the latter is a slightly more stable compound. Nevertheless, fuel can evaporate when exposed to warmer or sunnier weather, creating smog and other hydrocarbons.

Worse, it’s worth remembering that tailpipe emissions are only emitted when the engine’s running. Evaporating fuel, meanwhile, occurs 24/7 anytime there’s fuel in the tank, even when your car isn’t being driven. It’s been estimated that evaporative emissions account for around 20% of any vehicle’s lifetime pollution. Every single year, those tiny molecules of fuel could add up to nearly 30 gallons.

That’s 30 gallons of fuel leaking out of your gas tank in the form of uncontrolled vapor. However, it can be controlled, thanks to the advent of EVAP systems. In layman’s terms, an EVAP system creates a seal around the gas tank and your car’s entire fuelling system. With this seal in place, it’s harder for even tinier fuel particles and vapor to escape the tank, keeping it sealed tightly inside the tank.

What Are The Components Inside An EVAP System?

Before we look at how it all works, it might prove handy to get acquainted with the individual parts that make up the entire EVAP system. In particular, they include:

• Gas Tank – Naturally, this is where fuel is stored. A key design element is the inclusion of an expansion chamber at the top of the gas tank. On hot days, this allows gas or diesel to expand, or splash about while driving, without overflowing or leaking into the EVAP system.
• Gas Cap – It’s a lot more complex than it seems at first, mind you. Older cars feature gas caps that are fitted with a vacuum relief valve to vent out excess pressure. Newer gas caps though are sealed shut the whole way through, with no venting available.
• EVAP (Charcoal) Canister – You’ll notice it as a plastic or metal container, mounted somewhere inside the engine compartment, usually towards the back. Within, you’ll find one or a couple of pounds worth of activated charcoal, which helps to absorb fuel vapor like a sponge. When your engine is warmed, a canister purge valve opens, as the intake vacuum siphons the vapor into the engine to be burned.
• Liquid-Vapor Separator – It’s found near the top of the fuel tank. Primarily, the separator’s job is to ensure that liquid fuel doesn’t splash around and into the EVAP vent line that leads to the charcoal canister. Note, the EVAP canister is designed for fine fuel vapor, not whole fuel in liquid form. The latter can easily overwhelm the EVAP system, preventing it from storing excess vapor effectively.
• Vent Line – Sort of like a regular fuel line, but instead of liquid fuel, it carries vapor. It connects the liquid-vapor separator inside the gas tank to the EVAP canister where excess vapor is trapped.

How Does An EVAP System Work?

Interesting, creating a seal around the gas tank and fuelling system is much harder than it seems. An EVAP system still needs to allow air to vent into the gas tank to replace liquid fuel that’s pumped into the engine. Otherwise, and if the gas tank is sealed off completely, the fuel pump can create negative pressure inside the tank. With all that suction from the pump, this could collapse the gas tank.

In order, the EVAP system works like this:

1. As fuel is stored and left as-is naturally inside the gas tank, it’s still vulnerable to evaporating into a vapor form.
2. When it does, excess fuel vapor flows into the liquid-vapor separator, through the vent lines, and ends up inside the EVAP charcoal canister.
3. Here, that vapor will be safely stored, as the charcoal acts as a sponge to soak them in.
4. As you turn your car on, the engine will begin to warm up. When the engine reaches its operational temperature, the ECU (or ECM or PCM) opens up the canister purge valves.
5. Intake vacuum alone sucks up all the trapped fuel vapor captured in the charcoal and circulates it into the intake manifold.
6. Now, excess fuel vapor is mixed in with fresh air and liquid fuel pumped through the fuel injectors. This mixture helps to create a cleaner and more thorough combustion.
7. Typically, the canister purge valves only open when your car is in motion rather than idling. Once the EVAP system is no longer required, a canister vent valve closes and seals the entire system again.
8. The closing of the vent valves (and subsequent opening of the purge valves) are also a part of the EVAP system’s self-diagnostics. It creates a vacuum, which if not held properly, it’s determined that there’s a leak somewhere.

What Does A P0496 Error Code Mean?

As we highlighted earlier, an EVAP system is typically made to last a long time, and require minimal servicing. The charcoal canister, for example, uses charcoal that isn’t made to run or wear out over the vehicle’s lifespan. Nonetheless, the EVAP unit is still vulnerable to failure, as is the case with a P0496 error code. An accompanying message reads, “EVAP Flow During A Non-Purge Condition”.

Note those self-checks that we detailed before. The EVAP system regularly runs diagnostics tests on itself and reports back to the ECU. One of those tests involves making sure there isn’t an untimed or unwanted ‘purge flow’. The latter is defined as the final step in the process, where the excess fuel vapor is purged and flown into the engine. Before leading into the combustion chamber to be combusted.

But let’s say the excess fuel vapors flow through a vacuum when the system isn’t purging. Or, in other words, seeing fuel vapor being purged without the ECU telling it to. Not to mention the presence of a vacuum when there shouldn’t be any. In addition, if the vacuum is much higher and sticks around a bit longer. Should the fuel tank pressure (FTP) sensor detect this, your ECU flashes a P0496 error code

With your EVAP system’s programming, there’s something called a “purge-free cell”. These are those moments in which the EVAP system shouldn’t be purging fuel vapor into the engine. Should vapors continue to flow anyway during a purge-free moment, that’s when it triggers a P0496 error code. An ECU should be able to analyze changes in fuel trim when an EVAP purge has been activated.

What Causes A P0496 Error Code?

Oftentimes, the most likely culprit of a P0496 error code is a faulty canister purge valve (or solenoid) and/or vent valve that’s stuck open. Consequently, this leads excess fuel vapors to flow through into the engine freely, even when it’s not supposed to. Although, there are many other possible causes of a P0496 error code, such as:

• Bad fuel tank pressure (FTP) sensor, which is misreading vacuum levels during the EVAP system’s self-test.
• Leaking EVAP vacuum hose (aka vent line), as this can compromise the vacuum seal within the EVAP system and leak out fuel vapor.
• Clogged up EVAP charcoal canister, as it’s unable to sufficiently take in any more excess fuel vapor.
• Loose, missing, or broken gas cap, as it can’t adequately maintain a tight vacuum seal around the gas tank.
• Bad, damaged, frayed, loose, or corroded wiring, which may not be able to properly actuate the purge and vent valves to open or close.
• Short or open-circuit, once again failing to activate the canister purge and vent valves to open or close when needed. This issue is fairly prevalent in some Isuzu and Hyundai vehicles.
• Faulty EVAP system as a whole, as it leads to incorrect valve operation, as well as not properly sealing the fuel vapors in.
• Bad purge flow sensor, which is unable to properly detect when there’s an untimed purge flow. This is mostly applicable to Kia and Mazda vehicles that feature this sensor.

As noted earlier, the most common EVAP-related problem pertains to either a faulty purge or vent valve. There are two types of valves that are typically used. Mechanically-operated EVAP systems use a vacuum valve, which opens and closes depending on vacuum pressure. Meanwhile, electronically-actuated EVAP systems rely on solenoid valves, which open and close through applying voltage.

What Are The Symptoms Of A P0496 Error Code?

Unfortunately, any symptoms accompanying a P0496 error code are alarmingly subtle. You aren’t able to notice it most of the time. And, since the EVAP system is an enclosed unit, you might not be able to spot, hear, or feel any problems at all, let alone an issue with purge flow. Pay close attention, and it’ll show itself eventually, which include tell-tale P0496 symptoms such as:

• Check engine light (CEL) appears on the dashboard. Since you’re seeing a P0496 trouble code that’s been logged by the ECU, it’ll flash the check engine light to let you know that something’s amiss. You can then extract those error codes (such as P0496) using an OBDII diagnostics tool. As we’ll explain further down below, it’s best to rely on these error codes rather than looking at the symptoms.
• Your car might have difficulty with starting up, especially so once you’ve just filled up and brimmed the gas tank. Nevertheless, a car being hard to start and crank over is a typical symptom of many other issues and could be easily misinterpreted.
• Engine running richer than usual. This is another hard-to-spot symptom, which doesn’t manifest until some time has passed. A rich-running engine does have its own symptoms, though. These can include increased tailpipe emissions, poor fuel economy, rough idling, cylinder misfiring, poor performance, and more.
• Damage to the catalytic converters and exhaust system. This is a direct result of the engine running rich, as unburnt fuel is exhausted through the tailpipes. You might also notice black, sooty, and gas-smelling plumes of smoke. Just like the previous symptoms barring the CEL, it’s another sign that’s quite hard to notice. And even when you do, it could be confused for a myriad of other issues.

How Can You Diagnose A P0496 Error Code?

Initially, it may appear as though a P0496 error code is rather trivial, and doesn’t require immediate attention. True, purge flow issues will, at first, result in you having difficulties with starting the car up and turning over. However, it could quickly snowball from there. For starters, it’ll undoubtedly lead your engine to run in a richer air-to-fuel mixture than is ideally recommended.

A rich-running engine won’t be ruinous from the get-go. With that said, and ignore it for too long, it’ll begin to significantly place accelerated wear on your engine. Rich fuel mixtures will cause permanent and often irreparable damage inside the engine. At which point, you’ll have to spend thousands for a complex rebuild or replacement process. Therefore, a P0496 issue should be addressed promptly.

Although, knowing where to start with fixing a purge flow problem in your EVAP system can be fairly tough. Thus, a thorough diagnosis will be required, as you try to determine and pinpoint the points of failure that triggered a P0496 error code in the first place. You can begin by…

Step 1: OBD Scans And Quick Troubleshooting

Grab an OBDII diagnostics tool, and extract all possible error codes stored in the ECU. This includes a P0496 trouble code. If you find other error codes that point to more plausible issues elsewhere, try to diagnose and resolve those ones first before returning to P0496:

1. First, let’s use the OBDII scanner-slash-reader to clear out all the error codes, which resets the check engine light.
2. Now, go for a short test drive, and see if the check engine light reappears. Be sure to give the engine time to warm up appropriately, and take note of any other symptoms (if any) that you experience.
3. If it does, plug in your OBDII reader-slash-scanner to see if P0496 returns. If that’s the case, then you’ve confirmed that something is definitely wrong with your EVAP system or purge flow.
4. You can now begin with quick troubleshooting around the car. Remember, a check engine light can appear even for simple issues like a loose gas cap.
5. So, you can begin initial troubleshooting steps by checking to make sure the gas cap is tightened. It’ll emit a noticeable click once it’s firmly in place.
6. You could also walk around to the engine bay, and locate the EVAP (charcoal) canister. It’s often seen as a cylinder or rectangular box, placed far back near the firewall.
7. Once you’ve found it, inspect the connectors and wiring leading into it. Is there any visible damage, like fraying, corrosion, or if the connectors are a bit loose?

Step 2: Testing The (EVAP) Canister Purge Valve

Next up, we could start undertaking a physical diagnosis of the purge valve to see if it’s opening and closing properly. We’ll be assuming from the get-go here that the purge valve-slash-solenoid is stuck open:

1. First, make sure the engine is turned off, and your key is removed from the ignition.
2. Locate the EVAP charcoal canister, and pay close attention to where the hoses/lines are located. Find the hose that leads into the charcoal canister’s purge valve, from the gas tank.
3. Now, unplug all electrical connectors to the canister purge valve. This might set off other error codes, but we’ll ignore that for the time being.
4. With the electrical connectors detached, you can now slowly remove and unmount the vacuum hose.
5. Then, start the engine. You could use either an automotive vacuum gauge or just your finger, to see if any vacuum could be felt coming out of the purge valve, just where you removed the hose.
6. If a vacuum is present, then it’s a sign that the canister purge valve is leaking vapors when it’s not supposed to.
7. If a vacuum isn’t present, you can continue to the next step.
8. An alternative technique would be to remove the purge valve module entirely. Now, blow gently into it. Ordinarily, the valve should be closed and wouldn’t permit air to pass through. If you’re feeling the air on the other side, this validates the conclusion that the purge valves are faulty.

Step 3: Performing An EVAP Leak Test

This step is going to be far more advanced and may require some special tools to proceed. First, let’s take a peek at the fuel tank pressure (FTP) sensor to see if that one’s broken, instead:

1. Head over to your gas cap, and loosen or remove it.
2. With the engine turned off, plug an OBDII scanner. You’ll most likely need a more advanced unit.
3. Within said OBDII reader, find a menu where you can monitor the FTP sensor’s feedback in real-time.
4. If the FTP sensor shows there being a vacuum without the gas cap, then you can confirm that it’s the FTP sensor that has failed.

Next, we can conduct an EVAP leak test:

1. You’ll have to locate the vent lines leading to the EVAP vent valve.
2. Pinch the other end of this line/hose/tube, and pressurize the EVAP system with a smoke machine. The latter is dedicated to testing EVAP leaks.
3. If you notice smoke leaking out of any hoses or seals, then you know that you have an EVAP leak.
4. Try to patch up these leaks, and see if P0496 returns once more.

Step 4: Voltage Check On The FTP Sensor

If there isn’t a vacuum leak anywhere in the EVAP system, then you’ll have to evaluate the fuel tank pressure (FTP) sensor more closely:

1. Grab a multimeter (preferably a digital one), and attach one of the leads to the FTP sensor’s signal wire.
2. Once that’s done, you can either unmount the FTP sensor from the EVAP canister or disconnect the hoses leading to it. Just remember to leave the FTP sensor’s electrical wiring and connectors.
3. You can then use a hand pump to put a bit of pressure on the FTP sensor.
4. There should be a voltage reading outputted by the FTP sensor from this.
5. If there isn’t any reading, or if the voltage output is out of spec, then the FTP sensor needs replacing.

Final Thoughts (And Solutions) For P0496

As we’ve seen so far, our diagnosis points to three very likely suspects for triggering a P0496 trouble code. It’s either a bad canister purge valve, leaky vent lines and vacuum hoses, or a faulty fuel tank pressure (FTP) sensor. If that’s the case, then a prompt replacement of either part (following an in-depth diagnosis) will be necessary. Their approximate costs (including labor) are around:

• Canister Purge Valve/Solenoid Replacement – $80 to $200
• EVAP Vent Line (Vacuum Hoses) – $20 to $100
• Fuel Tank Pressure (FTP) Sensor – $280 to $330

As you can see, the costs for resolving a P0496 trouble code for good are relatively cheap. That’s when compared to the side-effects of delaying repairs, such as continually letting the engine run rich. That could, in time, cause significant internal damage and wear inside the engine. To a point where a simple repair won’t suffice. Remember that engine replacements or rebuilds cost thousands.

Beyond that, there are other consequences to not fixing an EVAP purge flow issue. Your ride won’t perform as well as it used to, with poor performance and difficulty starting. Additionally, you’ll now have to worry about a damaged catalytic converter, which costs at least $1,000. In all, a P0456 error, while not that serious at first, should be fixed ASAP to avoid pricier repairs down the line.

Approved Tools

These tools have been tried and tested by our team, they are ideal for fixing your car at home.