What this code means
P0455 may mean the EVAP system is seeing a leak condition where the system may not seal and hold vapor system pressure or vacuum as expected.
What the vehicle may do
- The driver may see a check engine light.
- The vehicle can often run normally.
- There may be a possible fuel vapor odor.
- EVAP readiness or emissions testing may be affected.
Possible fault areas
- Fuel fill cap sealing or installation may be involved.
- Fuel fill pipe or mating surface issues may be involved.
- EVAP hoses, tubes, or purge line connections may be involved.
- Purge valve or vent shut valve sealing may be involved.
- Canister, FTP sensor sealing, fuel tank unit gasket, or fuel tank sealing may be involved.
Diagnostic path
Opening and setup
On this 2010 to 2015 Honda Civic with the 1.8 gas engine, P0455 points to a possible EVAP system leak. In plain terms, the car may not be able to seal and hold the vapor system the way the ECM expects. The driver may only see a check engine light, and the car can often run normally. Broadly, possible areas can include the fuel cap and filler neck, EVAP hoses and tubes, purge or vent sealing, canister sealing, FTP sensor sealing, or fuel tank sealing. Before getting into the checks, record the captured data and any on-board snapshot. If possible, the best testing condition is less than a full tank of fresh fuel. To help leak detection, add 1 gallon of fresh fuel to the tank, as long as that will not fill the tank. Also, stay with the vacuum and pressure tests exactly as called out. Too much pressure or vacuum can damage EVAP parts or eventually damage the fuel tank.
Start at the fuel fill cap and filler neck
Start with the simplest sealing point: the fuel fill cap. Make sure the correct cap is installed. The cap must say TIGHTEN TO CLICK. When it is tight, it should still turn 1/4 turn, then click. If the cap is wrong or not tightened properly, replace the cap or tighten it, then move to verification. If the cap is correct and tight, keep going. Next, inspect the fuel fill cap seal and the fuel fill pipe mating surface. Also make sure the tether cord is not trapped under the cap. If the cap seal is missing or damaged, the fuel fill pipe is damaged, or the tether is caught under the cap, replace the fuel fill cap or fuel fill pipe as needed, then verify the repair. If none of that is present, the cap and fill pipe pass this part of the check.
Check the vapor recirculation tube and purge valve sealing
Next, turn the ignition switch to LOCK (0), and inspect the fuel tank vapor recirculation tube for a poor connection or damage. If the tube is not OK, replace the fuel tank vapor recirculation tube, or replace the fuel tank if needed, then verify the repair. If the tube is OK, move to the purge valve leak check. In the engine compartment, disconnect the vacuum hose, which is the purge line, from the EVAP canister purge valve. Connect a T-fitting, a vacuum gauge, and a vacuum pump/gauge, 0－30 inHg, to that hose. Slowly apply about 2 kPa (0.6 inHg, 15 mmHg) of vacuum to the hose. It needs to hold vacuum for 1 minute. If it holds, a leak is not detected at the EVAP canister purge valve, so continue. If it does not hold, replace the EVAP canister purge valve, then verify the repair.
Duplicate the fault with the scan tool
Once the basic leak points and purge valve sealing have passed, turn the ignition switch to ON (II). Clear the DTC with the HDS, then run the EVAP FUNCTION TEST with the HDS. If the test result is OK, treat it as an intermittent failure and the system is OK at this time. Check for poor connections or loose terminals at the FTP sensor, the EVAP canister purge valve, the EVAP canister vent shut valve, and the ECM/PCM. If the EVAP FUNCTION TEST is not OK, the failure is duplicated, so continue with the isolation tests.
Isolate the canister side from the rest of the EVAP system
For the next isolation check, turn the ignition switch to LOCK (0). Disconnect the fuel tank vapor recirculation tube from the EVAP canister, and plug the EVAP canister port. Remove the T-fitting, and connect a vacuum pump/gauge to the vacuum hose. Turn the ignition switch to ON (II), then use the HDS inspection menu to select EVAP CVS ON. Apply vacuum to the hose until the FTP reads 1.90 V (－0.59 inHg, －15.1 mmHg). Be careful not to exceed that vacuum, because the FTP sensor can be damaged. Monitor the FTP SENSOR in the data list for 1 minute with the HDS. If the voltage increases more than 0.2 V (0.1 inHg, 2.5 mmHg), continue to the vent shut valve operation check. If the voltage does not increase more than 0.2 V (0.1 inHg, 2.5 mmHg), skip ahead to the fuel fill pipe and fuel vapor return pipe inspection.
Check EVAP canister vent shut valve operation
For the vent shut valve operation check, select EVAP CVS OFF in the HDS inspection menu. Disconnect the fresh air hose from the EVAP canister, and plug the EVAP canister vent shut valve ports. Turn the ignition switch to ON (II), then select EVAP PCS ON with the HDS. Apply vacuum to the hose until the FTP reads 1.90 V (－0.59 inHg, －15.1 mmHg). Again, do not exceed that vacuum, because the FTP sensor can be damaged. Then select EVAP PCS OFF, and monitor the FTP SENSOR in the data list for 1 minute. If the voltage increases more than 0.2 V (0.1 inHg, 2.5 mmHg), the EVAP canister vent shut valve is OK, and the next check is the purge line. If the voltage does not increase more than 0.2 V (0.1 inHg, 2.5 mmHg), replace the EVAP canister vent shut valve, then verify the repair.
Finish the leak path checks
If the vent shut valve passed, visually inspect the EVAP canister purge line. If that line is not OK, reconnect or repair the EVAP canister purge line, then verify the repair. If the purge line is OK in this branch, replace the FTP sensor O-ring, the EVAP canister vent shut valve case and O-ring, and the EVAP canister, then verify the repair. If the earlier isolation check sent you toward the fill-side inspection, select EVAP CVS OFF with the HDS and check for looseness or damage at the fuel fill pipe and the fuel vapor return pipe. If those parts are not OK, repair or replace the damaged parts, then verify the repair. If they are OK, check the fuel tank unit base gasket and the fuel tank, then verify the repair.
Verify the repair
Keep verification separate from the testing path. Turn the ignition switch to LOCK (0), install the parts in the reverse order of removal, then turn the ignition switch to ON (II). Reset the ECM/PCM with the HDS, do the ECM/PCM idle learn procedure, and run the EVAP FUNCTION TEST with the HDS. If the result is OK, troubleshooting is complete. If the result is not OK, check for poor connections or loose terminals at the FTP sensor, the EVAP canister purge valve, the EVAP canister vent shut valve, and the ECM/PCM, then restart the diagnostic path at the fuel cap. The takeaway: on P0455, prove the easy sealing points first, control the EVAP valves with the scan tool, watch the FTP response, and only make the repair that matches the branch you proved. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0455 is best handled by proving the sealing path in order, starting with simple cap and filler checks, then using scan-tool valve commands and FTP sensor response to narrow the leak area.
For more guided automotive diagnostics, visit STEP Diagnostics.
