P0455: P0455 Diagnostic Guide

What this code means

P0455 may mean the EVAP system is not sealing or the PCM may not be seeing expected purge flow during the EVAP monitor.

What the vehicle may do

The vehicle may drive normally with the check engine light on.
The vehicle can have an emissions inspection concern.
Fuel odor may be possible if there is a larger vapor leak.

Possible fault areas

Possible fuel cap or capless filler sealing issue.
Possible EVAP hose, line, filler pipe, canister, vent, tank, or connection leak.
Possible vapor-path restriction.
Possible FTP sensor signal concern.
Possible EVAP canister purge valve operation or sealing concern.

Diagnostic path

Opening context

On this 2015 to 2025 Ford F-150 with the 3.5 EcoBoost, P0455 is an EVAP large leak code. In plain terms, the PCM may be seeing no purge flow, a large fuel vapor leak, or more than one leak during the EVAP monitor. The truck may drive normally, but it can have a check engine light and may fail an emissions check. The broad fault areas are possible sealing problems at the fuel filler, possible damaged or misrouted EVAP plumbing, possible restriction in the vapor path, possible fuel tank or filler damage, or a purge valve or FTP sensor concern. Work it in order, because this code is about proving whether the system can seal and whether purge flow is actually responding when commanded.

Start with the system checks and confirm the code path

Start with the basic system checks, then run the EVAP test with the scan tool: the KOEO EVAP leak test on hybrid applications, or the PCM evaporative emissions system test on the others. If that scan tool test is not available on the vehicle, keep moving to the next check. After that, check DTCs. If other codes are present, check what they mean first, but for this episode we are following the P0455 path. With P0455 present, continue into the EVAP visual inspection.

Initial EVAP visual and purge check

Visually inspect the capless filler or fuel cap area, the EVAP lines, and the hoses for incorrect connections or damage. Then run the purge valve check in the EVAP toolbox on the scan tool if it is available. If you find a concern here, repair it as needed, clear the PCM DTCs, repeat the self-test, and rerun the EVAP system test. If the visual and purge check do not show a concern, continue to the fuel filler sealing check.

Fuel filler cap or capless filler inspection

Next, check the filler sealing area without disturbing it at first. On a capped system, make sure the tether is visible and free to move, then look for a missing, loose, or damaged fuel cap. On a capless system, inspect the inlet for debris that could keep it from sealing, and check the capless filler pipe for damage. If it is capless, install and remove the supplemental fueling adapter five times to help clear debris from the seal, then repeat the check. Do not try to call this leak with soap solution around the filler or with a hydrocarbon analyzer; that approach is not recommended here. If there is a concern, repair it as needed and verify it with the Phase 1 leak verification pressure test. If there is no concern, move deeper into the EVAP system inspection.

Inspect for restrictions and damaged hard parts

With the ignition off, inspect the vapor path for restrictions and damage. On this 3.5 EcoBoost path, that means looking for a restricted vacuum hose between the purge valve and the engine intake manifold, a restricted vapor tube between the purge valve and the leak detection control module if equipped, a damaged fuel filler pipe, and a damaged fuel tank. If you find a concern, repair it as needed, verify with the PCM evaporative emissions system test, clear the PCM DTCs, and repeat the self-test. If there is no concern, go to the FTP sensor check.

Check FTP sensor reading with the system vented

Now open the fuel system to atmosphere for the FTP reading. On a capless filler, install the supplemental refueling adapter to open the filler pipe. On a capped system, remove the fuel cap. Turn the ignition on, access the PCM, and monitor the FTP_V PID. On the listed hybrid applications, the FTP reading should be between 1.409 - 1.54 volts. On all other applications, the FTP reading should be between 2.1 - 2.9 volts. If it is in range, continue to the purge valve operation test. If it is not in range, install a new FTP sensor, repeat the test, verify the result, then continue to the Phase 1 leak verification pressure test.

Command the vent and purge valves and watch response

For the purge valve operation test, turn the ignition off, reinstall the fuel cap or remove the supplemental adapter, then start the engine. Command the EVAP canister vent valve closed by setting EVAPCV to ON, which is 100% duty cycle. Monitor FTP_V, then command the purge valve open in steps until EVAPCP reaches 100%. You are looking for a real response: the FTP PID drops, engine RPM changes, or the engine stalls. If one of those happens, the purge valve is opening and you move to the leak test setup. If it does not happen, install a new EVAP canister purge valve, repeat and verify the test, then perform the Phase 1 leak verification pressure test.

Hook up the leak detector safely

Before disconnecting fuel tubes or fuel system components, relieve fuel system pressure, because fuel can stay under high pressure even with the engine off. Handle fuel carefully and be ready for spillage. For the leak detector, use the Evaporative Emission Leak Detector With On Board Nitrogen Generator 218-522C or an equivalent tool. The fuel level in the fuel tank must be less than 85% full for this equipment. Do not apply smoke to the EVAP leak detection control module. With the ignition off, connect the leak detector, disconnect the fuel vapor-to-canister line at the purge valve, and connect a suitable hose adapter to that line. If the hookup is complete, go either to leak detection or to verification, depending on where you are in the diagnostic path. If the hookup is not complete, pause and correct the setup before testing.

Smoke test for the leak

For Phase 2 leak detection, remove the fuel cap or install the supplemental refueling adapter, connect the supply hose, and set the leak detector to smoke. Turn the ignition on and command the canister vent valve closed with EVAPCV ON at 100% duty cycle. Start the smoke supply and first make sure the connection at the purge valve line is not leaking. If smoke does not come out of the filler after the system is pressurized, command the vent valve open until smoke is seen at the vent valve, then command it closed again. Once smoke exits the filler, install the fuel cap or remove the supplemental adapter, keep smoke going into the EVAP system, and use the halogen spotlight along the EVAP path to find smoke escaping. Check the hoses, purge and vent valve areas, canister, fuel tank, filler pipe, fuel tank area, and the filler cap or capless inlet. Wiggle components and connections while watching for smoke. Remember that smoke and airflow will not pass through liquid fuel, and smoke under pressure may not reach a filler-pipe leak between the check valve and the filler opening. If you do not find smoke, visually inspect that filler area again. If the leak is found, repair as needed, reconnect everything, run the EVAP system test for repair verification, and continue to the pressure verification test. If the leak is not found, reconnect everything and still run the Phase 1 pressure verification test.

Verify the system holds pressure

Keep verification separate from the leak search. For the Phase 1 leak verification pressure test, calibrate the flowmeter using the 0.020 standard, connect the supply hose through the adapter, and set the leak detector to the 0.020 standard. Turn the ignition on, command EVAPCV ON at 100% duty cycle to close the canister vent valve, and start the pressure supply. The flowmeter ball should start at the top while the system pressurizes. Keep pressurizing until the ball stops descending, then compare it to the red pointer flag. Below the indicator flag is a pass. Above the indicator flag means the EVAP system still has an unacceptable leak. For P0455, if the system passes this pressure test, continue to the purge valve leak test. If it does not pass, go back to the smoke leak detection step.

Final purge valve leak check and closeout

Because this is P0455, a system that passes the pressure verification still gets the EVAP canister purge valve leak test. Run the purge valve check in the EVAP toolbox if it is available. If it is not available, remove the purge valve, connect a suitable adapter to the fuel vapor-to-canister connection, connect the supply hose, and start the pressure supply. Let the flowmeter ball descend until it stops, then compare it to the red pointer flag. Below the indicator flag means the purge valve passed the pressure test. Above the indicator flag means the purge valve has an unacceptable leak. If the purge valve passes, the test passed and no concerns are present; clear the PCM DTCs and repeat the self-test. If it does not pass, install a new EVAP canister purge valve, run the EVAP system test, clear the PCM DTCs, and repeat the self-test. The takeaway: do not jump straight to parts on a P0455. Prove the filler seal, prove the vapor path, prove the FTP signal, prove commanded purge response, then verify the system and purge valve can hold pressure. For more diagnostic training, visit stepdiagnostics.com.