Real world EVAP: How to diagnose a P0455 code

Can’t find the smoke? Don’t assume there’s no 'fire.'

If this is the case, the first step is to visually check the fuel cap seal to make sure the fuel cap is fitting correctly. You want to make sure the gasket is in good condition and the filler neck where it meets the cap is not too rusty. If there is any doubt, replace the cap.

A smoke machine with a flow meter is a must for this job. Too many times, when a technician sees a DTC P0455, he puts the smoke into it and starts looking around. But what if there’s no smoke to be found?

This is where the flow meter comes in. You want to make sure there is a leak before you go looking for one. A large leak will show up very easily with a smoke machine flow meter. This should take less than two minutes once you have sealed the system by closing the vent solenoid with your scan tool or PowerProbe. If the system is good, the ball should steadily descend to the bottom of the flow meter to 0.020” or even 0.010” and less. If the ball in the flow meter never drops to the bottom and stays in the upper section, you have a leak. As one trainer is fond of saying: “If the ball is floatin’ it’s time to start smokin’!”

Most of the time, the smoke will lead you right to the location. But sometimes, this location is hard to get to or see, like the top of the tank, or under a shield. Sometimes the fuel level of the tank can prevent a leak from being seen. What approach should be taken if the smoke can’t been seen?

The UV Dye approach

Any machine designed to work with UV dye and the appropriate UV light and goggles will help you find the dye spot that’s left behind like a fingerprint to show you the exact location of the leak. Be sure the UV dye is OEM-approved or the dye might corrode the diaphragms in the EVAP solenoids, creating future leaks.

To be quick, accurate and avoid comebacks, you should run the smoke for the full 5-minute cycle if the source of the smoke is hidden and requires disassembly. Why? Like all UV dye, it takes time to deposit. Smoking a “hidden” leak for less than five minutes usually isn’t a shortcut; it can actually make finding some leaks impossible.

Make sure your UV light matches the UV dye. To test it, pull the dipstick out of your smoke machine and shine your UV light on it. If you have two or three different lights, try them all to see what works best. The right UV light can make all the difference. Lastly, look at likely spots and connections. If necessary, remove parts that might be in your way from observing critical EVAP components that might be leaking.


An emissions analyzer can be used to check for everything from a leaking gas cap to a corroding FTP. It will indicate an increase in hydrocarbons (HC) if there is a fuel or fuel vapor leak.

Place the car on a lift in a closed bay, without any fans. Take the probe and work from one end of the car to the other, following EVAP lines and components from end to end. Be sure to move the probe slowly to allow time for the machine to update and give an accurate HC count. Don’t forget to get the probe above the gas tank to check spots you cannot see, and work your way into areas where the EVAP canister is.

To simulate what kind of numbers you would see, loosen the gas cap, wait a minute, and run the probe near the gas cap until the numbers spike. This will indicate how close you need your probe and what kind of HC count you should expect.

Try to test around critical leak points, depending on the manufacturer of the vehicle. Don’t immerse the analyzer probe in liquid fuel. If you do, it will break.

There are different ways to diagnose an EVAP problem. A shop can use just a vacuum pump and a generic OBDII scan tool or other tools such as an emissions analyzer.

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