P0455: P0455 Diagnostic Guide

What this code means

P0455 may indicate the PCM is seeing an EVAP system problem that can look like no purge flow, a large vapor leak, or multiple vapor leaks.

What the vehicle may do

The vehicle may turn on the check engine light.
The driver may notice a fuel odor.
There may be audible vacuum noise near the engine compartment, EVAP canister, or fuel tank.

Possible fault areas

Possible fuel filler cap or capless filler inlet sealing concerns.
Possible EVAP hose, vapor line, fuel filler pipe, fuel tank, or canister area concerns.
Possible purge or vent control concerns.
Possible FTP sensor signal concern.

Diagnostic path

Opening context

On this F-150 5.0L, P0455 means the PCM may be seeing an EVAP problem that looks like no purge flow, a large vapor leak, or multiple vapor leaks while the EVAP monitor is running. The driver may only see a check engine light, but you may also get a fuel odor or hear vacuum noise near the engine compartment, EVAP canister, or fuel tank. The broad fault areas can include the filler neck or capless inlet, EVAP hoses and vapor lines, purge and vent control, the fuel tank area, and the FTP sensor signal. The goal here is not to guess at parts. Work the path in order and prove whether the system is sealed, whether purge is actually working, and whether the pressure signal makes sense.

Start with scan-tool checks

Start with the basic system checks, then run the scan-tool EVAP test if it is available. On this path, whether that first EVAP test passes or fails, continue by checking which codes are present. If other codes are present, check what they mean first, but for P0455 staying active, continue into the EVAP visual inspection path. If no listed EVAP codes are present, you are no longer on this P0455 code path and should treat it as a symptom-based complaint instead.

Initial EVAP inspection

Next, look over the easy leak points before you start forcing the system with test equipment. Inspect the capless fuel tank filler pipe or fuel cap area, the EVAP lines, and the hoses for damage or incorrect connections. If the scan-tool purge check is available, run it here. If you find a concern, repair it as needed, clear the PCM codes, repeat the self-test, and run the EVAP test again. If the inspection does not find anything for P0455, keep moving down the large-leak path.

Fuel filler cap or capless inlet

Now focus on the fuel filler sealing area. If the truck has a fuel filler cap, inspect it before disturbing it. Make sure the tether is visible and free to move, and check for a missing, loose, or damaged cap. If it has a capless fuel tank filler pipe, inspect the inlet before disturbing it. Look for debris that can hold the seal open or for damage at the inlet. Use the supplemental fueling adapter five times to help clear debris from the seal, then repeat the check. If a concern is found, repair it as needed and verify the repair with the leak verification pressure test. If there is no concern at the filler area, continue to the next inspection.

EVAP plumbing inspection

With the ignition off, inspect the rest of the EVAP plumbing called out on this path. Check for a restricted vacuum hose between the EVAP canister purge valve and the intake manifold, a restricted fuel vapor tube between the purge valve and the EVAP leak detection control module if equipped, a damaged fuel filler pipe, and a damaged fuel tank. If you find a concern on this 5.0L application, repair it as needed, run the PCM EVAP system test, clear the PCM codes, and repeat the self-test. If that inspection is clean, move to the FTP sensor voltage check.

FTP sensor voltage check

Open the tank to atmosphere for the FTP check. On a capless filler, install the supplemental refueling adaptor to open the capless fuel tank filler pipe. On a capped system, remove the fuel filler cap. Turn the ignition on, access the PCM, and watch the FTP_V PID. This check has two voltage windows: the listed hybrid applications use an FTP PID reading between 1.409 - 1.54 volts, and all others use an FTP PID reading between 2.1 - 2.9 volts. For this 5.0L non-hybrid path, use the all-others range. If the reading is in that range, continue to the purge valve functional test. If it is not in range, the path calls for installing a new FTP sensor, then repeating the test and verifying the result before moving into leak verification.

Purge valve functional test

For the purge test, turn the ignition off, reinstall the fuel cap or remove the supplemental refueling adaptor, then start the engine. Access the PCM controls. Command the EVAP canister vent valve closed by commanding EVAPCV to ON at 100% duty cycle. Then monitor FTP_V while incrementally commanding the EVAP canister purge valve open with EVAPCP up to 100%. You are looking for the FTP PID to decrease, the engine RPM to change, or the engine to stall, which indicates the purge valve is opening. If you get that response, continue to test-equipment hookup. If you do not get that response, the path calls for installing a new EVAP canister purge valve, repeating and verifying the test, then carrying out Phase 1 leak verification.

Hook up leak test equipment safely

Before disconnecting fuel tubes or fuel system components, relieve fuel system pressure. Fuel can remain under high pressure even with the engine off, and spilled fuel can ignite on hot parts or other ignition sources, so handle fuel carefully. Also, do not apply smoke to the EVAP leak detection control module. For the leak equipment setup, the fuel level must be less than 85% full when using the Evaporative Emission Leak Detector With On Board Nitrogen Generator 218-522C or equivalent. With the ignition off, connect the leak detector according to the tool setup, disconnect the fuel vapor-to-EVAP canister line at the purge valve, and connect a suitable hose adapter to that line connection. Smoke and airflow from this equipment will not pass through liquid fuel, and liquid fuel may be present in the filler pipe. If the hookup is complete, go to smoke leak detection for finding a leak, or to pressure verification when you are verifying a repair.

Phase 2 smoke leak detection

For Phase 2 leak detection, remove the fuel filler cap or install the supplemental refueling adaptor. Connect the supply hose to the adapter and set the leak detector to SMOKE. Turn the ignition on, close the canister vent valve by commanding EVAPCV ON at 100% duty cycle, and start the smoke supply. Make sure the connection at the fuel vapor-to-EVAP canister line near the purge valve is not leaking. If smoke does not come out of the filler pipe after the system is pressurized, command the vent valve open. Once smoke is seen at the canister vent valve, command the vent valve closed again. Then install the fuel cap or remove the supplemental adaptor once smoke exits the filler pipe, and keep applying smoke. Use a halogen spotlight along the EVAP path and look for smoke escaping at the leak. Check the hoses, purge valve, vent valve, canister, tank, filler pipe, tank area, and filler cap or capless inlet. Wiggle components and connections while watching for smoke. Do not rely on soap solution at the fuel filler area or a hydrocarbon analyzer for this leak decision. If the smoke test finds a leak, repair it as needed, reconnect everything that was disconnected, run the scan-tool EVAP test, and then move to pressure verification. If no leak is found, reconnect the components and still run Phase 1 pressure verification.

Phase 1 leak verification pressure test

Keep verification separate from leak searching. For Phase 1, calibrate the flowmeter using the 0.020 standard according to the tool setup. Connect the supply hose to the vehicle through the suitable adapter and set the leak detector to the 0.020 standard. Turn the ignition on, close the canister vent valve by commanding EVAPCV ON at 100% duty cycle, and start the pressure supply. Continue pressurizing the EVAP system until the ball stops descending. Once it stops, compare the ball position to the red pointer flag. If the measurement is below the indicator flag, the system passed the pressure test. If it is above the indicator flag, the EVAP system still has an unacceptable leak, so go back to smoke leak detection instead of guessing.

Purge valve leak verification and final self-test

After the EVAP system passes pressure verification for P0455, finish with the EVAP canister purge valve leak test. Run the scan-tool purge check if it is available. If it is not available, remove the purge valve, connect a suitable hose adapter at the fuel vapor-to-EVAP canister connection, connect the supply hose, start the pressure supply, and pressurize the purge valve until the ball stops descending. If the measurement is below the indicator flag, the purge valve passed the pressure test. If it is above the indicator flag, the purge valve has an unacceptable leak and the path calls for installing a new EVAP canister purge valve, running the EVAP test, clearing the PCM codes, and repeating the self-test. If the purge valve passes, the test has passed with no concerns present; clear the PCM codes and repeat the self-test to confirm P0455 stays gone.

Takeaway

Bottom line: for P0455 on this F-150, prove the filler area first, check the EVAP plumbing, validate the FTP signal and purge response, then use smoke detection and pressure verification to separate an actual leak from a control or sensor issue. For more diagnostic training, visit stepdiagnostics.com.