Real world EVAP: How to diagnose a P0455 code

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

Vehicle Application:

• 2004 Grand Prix 3.8

Avg. Reported Mileage:

• 90,000 miles

Direct Hits:

• 372575

Customer Concern:

Code P0455 for a gross leak keeps coming back. The technician replaced the fuel cap and it came back. The technician then smoke tested system and cannot find any leaks.

Tools Used:

  • Scan tool
  • Smoke machine
  • Flow meter
  • Ultraviolet dye solution
  • UV light
  • Goggles



  1. Apply vacuum to the disconnected tank hose at the evaporative canister.
  2. Test for fuel cap seal to tank filler neck.
  3. Test for leak at Fuel Tank Pressure (FTP) sensor.
  4. Replace FTP sensor as per TSB 04-06-04-025A.

A customer rolls in with a "check engine" light. You scan a P0455, an engine code that indicates a large evaporative emissions (EVAP) leak. Your first move: check the gas cap.

If the gas cap has been sealed properly and the code still comes back on, the EVAP system needs to be inspected for leak points. This can be due to cracked hoses or parts (like the canister) or an area in the car that has been damaged by rust, such as the fuel filler neck or the gas tank. The challenge is often not in fixing the leaks, but finding them.

A P0455 leak is a common EVAP code, since any defect in the EVAP system can set it off, aside from internal diaphragm leaks in any EVAP-related solenoids. On a GM, for example, a P0455 can be set off by a vent solenoid, fuel tank pressure (FTP) sensor, the charcoal canister, vapor lines, hose connections, filler neck, the fuel tank itself – anything that can cause a leak of more than 0.020”.


If you are using a traditional scan tool, you have to find all the necessary diagnostic information in an outside database. Depending on what subscription you have, you can search by technical service bulletin (TSB) or by searching the code itself to get the diagnostic procedure from a repair information system.

Some scan tools have all this information built in. With online access, the web browser in these newer scan tools gives access to the same repair information systems available on a regular desktop. With this technology, the diagnosis can be accomplished in a matter of minutes: get the code, search the database for a cause, and find a solution without leaving the vehicle.

The P0455 code does not mean a large leak is detected. The code simply indicates the evaporative system is not able to pull the fuel tank pressure down to 0.8V. Therefore, since it cannot meet its target vacuum, it considers this a “gross” leak.

Some techs believe that small leaks are easier to diagnose using a vacuuming tool rather than smoke. One method is to block one piece of the system at a time, such as a charcoal canister, fuel tank and sections of EVAP lines, then pull that part of the system to vacuum and check its leak rate. This approach requires a vacuum pump and a lot of time: blocking each section of the EVAP system, section by section, and creating a vacuum with the pump.

For example, if you suspect the tank hose is faulty, disconnect the tank hose at the evaporative canister and apply a vacuum pump to the tank hose. The purpose is to make sure that anything connected to and including the tank hose does not have a leak. This is accomplished by vacuuming all the air out and seeing if the pump can hold vacuum. If there is a leak, pressure will change.

If the tank hose leaks, the leak will usually be at the fuel pump gasket. The leak can be very small, with no liquid escaping, but big enough for vapor to escape, setting off the EVAP code. Moisture above the tank can also cause rust at the top, which can be a leak point as well.


The leaks can easily occur some place that isn’t easy to get to, such as at the fuel pump that was referenced earlier. How can you save time and find the leak without spending all day? The scan tool only indicates a P0455 diagnostic trouble code (DTC), a large leak. But the scanners do not think for us yet. You might not even have a large leak at all, as some OEs set the DTC whenever there is a leak of any size. A restriction in the EVAP system can also trigger this DTC.

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