Understanding EVAP System Diagnostics and the Tools You’ll Need to Get the Job Done

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Digging into the EVAP system, FTP sensor problems, and related components requires a bit more knowledge than other maintenance tasks. You also need a more advanced, reliable tool, and we explore why.

• Exploring Why EVAP System Issues Are Easy to Misdiagnose
• What Basic Scanners Can’t Show You and Why You Need Better Tools
• What About a Smoke Machine?
• What To Look for When Choosing an EVAP Diagnostics Tool
• What’s Next? — Diagnosing FTP and EVAP Failures
• FAQs: Understanding EVAP System Diagnostics and Basic Questions

Tucked away inside the fuel tank of your vehicle, or sometimes mounted on top of the fuel pump, is a unique pressure sensor. It’s designed to read the pressure of the fuel system to monitor its operating health and identify potential leaks — evaporative leaks. Collectively, it’s part of an internal configuration called the Evaporative Emissions System, or EVAP for short.

The sensor communicates with the vehicle’s engine computer, and when a leak or malfunction is detected, the check engine light is activated. But like any vehicle system error, mission-critical process or not, the check engine light is generalized. That means when it goes on, it’s not always apparent why. Since fuel tank pressure sensor issues are more than just a check engine light — they’re often the beginning of a frustrating emissions system troubleshooting puzzle — you’ll need to enlist a little extra help to find the root cause.

That’s why manufacturers use the Diagnostic Trouble Code (DTC) system, which can be easily read using an OBD2 scanner. If you see code readouts like P0451, P0452, or P0453, for example, you’re in the EVAP zone. And to diagnose it right, you’ll need more than a basic code reader.

Understanding the evaporative emission (EVAP) system and diagnosing it accurately requires not only the right data, but also the right tools — the right scanner.

Exploring Why EVAP System Issues Are Easy to Misdiagnose

While the fuel pressure sensor may be measuring a relatively straightforward operating element of the engine — fuel fumes and emissions — the system itself is much more complex. A malfunction or problem can reside with a number of related systems, and is not merely the fault of the sensors.

When the tank pressure reaches a certain level or drops below the threshold, the sensor sends a signal to the electronic control module (ECM) or the vehicle’s primary computer. That signal is used to determine how much fuel is left in the tank and adjust the engine’s running conditions accordingly. So problems with the sensor can cause rough engine idling, poor performance or low fuel efficiency, failed emissions tests, hard engine starts, and sometimes odd smells, like the strong smell of fuel or burning gas.

Some common elements or components of the EVAP system are:

• Fuel tank pressure sensor
• Purge solenoids and vent valves
• Charcoal canisters and vapor lines
• Electronic controls and circuitry
• Emissions leak detection monitors

The reason why this list of elements is important is that a small issue with any one of these systems can trigger the check engine light and cause a malfunction or anomalous operation. Even a minor leak or a stuck valve can fire off the error warnings, resulting in a vague code or warning light that sends you into a frenzy.

It’s a very complex system that you can’t easily take apart or examine, so you could realistically spend hours upon hours troubleshooting an error code, even for something ridiculously simple, like a stuck valve.

To make matters worse, fixing an evaporative leak may not always require removing components like the fuel tank. But replacing or fixing a sensor usually does. A misdiagnosis can lead to the costly and time-consuming disassembly of systems that end up not even being related to the source of the problem.

What Basic Scanners Can’t Show You and Why You Need Better Tools

The complex nature of the EVAP system and the multiple ways in which it can be serviced or maintained call for a more precise, reliable tool that can truly speed up the troubleshooting process. By now, you’ve probably figured out that means using a more advanced OBD2 or diagnostics scanner.

While nearly all scanners on the market can read and report the related trouble codes, like a P0451 flag with a short description, you’re missing half the picture with that information. It’s a surface-level diagnosis that could lead to some timely mistakes.

For instance, you won’t be able to see:

• Real-time FTP sensor readings
• Live purge and vent command states
• System monitor readiness
• Related fuel trim or Mass Air Flow (MAF) sensor anomalies that point to the real issue(s)

The real-time fuel tank pressure sensor voltages can help you determine whether it's a sensor malfunction, if there’s a leak, or if something else is the culprit.

What About a Smoke Machine?

It’s true that mechanics and DIY enthusiasts often use a smoke machine to locate vacuum and emissions system leaks. Essentially, they push thick smoke through the system pipes, valves and tubes, and it will exit where there are holes or leaks. They’re good to have, but you don’t need one. You can actually get surprisingly far without a smoke machine, provided you have alternative tools.

If you have an advanced scanner that can monitor system behavior before and after system activations, you’re all set. The Innova 5610 is our recommendation for this.

With the 5610, you can:

• Monitor FTP values during purging and venting events.
• Compare live data with expected readings to discern issues.
• Confirm successful text cycles with readiness indicators.

What To Look for When Choosing an EVAP Diagnostics Tool

Whether you go with the Innova 5610 or another scanner, a proper diagnostics process must cover the following:

• Displaying live FTP sensor voltages in real-time.
• Reveal purge valve command activity and status alerts.
• Reports on readiness monitor states, especially EVAP statistics.
• Provide access to manufacturer-specific codes and subsystems for your particular vehicle.

Without these features, you’re navigating in the dark. Your best option in that case is to head to a local repair shop and let them handle the repairs.

What’s Next? — Diagnosing FTP and EVAP Failures

Ready to take that road trip you’ve always wanted? First, we have to get your vehicle up and running.

Now that you understand how diagnosis works for FTP and EVAP system failures, you’re going to need to get your hands on the appropriate tools. To see why the Innova 5610 is your best option, read our comparison of the Innova 5610 versus 5210 for EVAP diagnostics.

FAQs: Understanding EVAP System Diagnostics and Basic Questions

After reading our guide, you may still have some questions about your vehicle’s EVAP system, related diagnostics, and the tools you can use to troubleshoot. We’ll do our best to answer them here.

Why do you need a scanner tool in the first place?

While we realistically covered this in the guide, it helps to break it out separately for those who like to skim through lengthy coverage. A check engine light can mean virtually anything. Even something as simple as a loose gas cap can trigger the light, leaving the vehicle owner stumped.

You use a scanner tool, or an OBD2 scanner, specifically, to read the diagnostic trouble codes associated with the check engine light. But again, basic code readers won’t give you the nuanced information you need to make a decision when it is an advanced malfunction — like anything related to the EVAP system.

Is there a problem you should always check before assuming there’s an EVAP failure?

Yes, actually. Before spending any money on repairs or digging into the finer elements of the system, always inspect the gas cap. A faulty gas cap, loose gas cap, or problem with this component can cause EVAP error codes to show.

Always make sure you properly tighten the gas cap. Listen for a click when tightening the cap. If there’s no click, it may not be tight enough. Otherwise, check for cracks or damage in the cap. If you notice any issues, you may need to replace the OEM gas cap — which is much cheaper than digging into the actual EVAP system.

What else do you need besides an OBD2 scanner?

It really depends on the problem at hand. An OBD2 scanner is always the best place to start, especially an advanced pro-grade tool like the Innova 5610. If you’re dealing with a parasitic draw or electrical problems, you might also want to use a digital multimeter to read circuit patterns. An FTP sensor concern might require a pressure-sensitive tool to check system pressures, and so on.

What are some common EVAP-related diagnostic trouble codes?

When the check engine light triggers by a fault in the EVAP system, it’s commonly related to one of the following codes:

• P0440: This is a general EVAP system malfunction.
• P0442: This means a small leak was detected. As you’ll notice, there are varying codes for the size of the leak.
• P0455: A large system leak was detected.
• P0456: A very small leak was detected somewhere.
• P0496: There is an EVAP purge flow fault.

If you encounter any of these codes, you must inspect the system further to get to the bottom of the issue.

How long does it take for the check engine light to go away after repairs?

Depending on the issue, it may take a few drives before the check engine light automatically turns off, provided the repair was successful. As an estimate, that could be anywhere from 20 to 100 miles.

But here’s another benefit of having an OBD2 scanner tool handy. You can use these tools, including the Innova 5610, to reset the code by manually clearing it and turning off the check engine light in the process.

Interested in learning more? Learn more about Innova 5610 versus 5210 for EVAP Diagnostics.

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