P0455: Evaporative Emission System Large Leak Detected

What this code means

P0455 may mean the EVAP system is not sealing well enough for the vehicle to build or hold vacuum during its leak check.

What the vehicle may do

The vehicle may run normally with the check engine light on.
There may be a fuel odor in some cases.
The EVAP monitor may not complete until the fault is corrected.

Possible fault areas

A possible loose, missing, damaged, or incorrect fuel fill cap on vehicles equipped that way.
Possible damage at the fuel filler neck seal surface.
Possible EVAP hose, pipe, canister, purge, vent, or connection leakage.
Possible purge-path restriction.
Possible fuel tank pressure sensor signal concern.

Diagnostic path

Code overview and setup

On this 2014 to 2018 GMC Sierra 1500 with the 5.3 gas engine, P0455 means the EVAP system may be acting like it has a large leak. The truck may still run normally, but the check engine light can be on, and depending on the fault, there may be a fuel odor or an EVAP monitor that will not complete. Broadly, think about possible leakage at the fuel fill area, EVAP hoses and connections, the canister side of the system, the purge path, or the fuel tank pressure signal. Start with the basic system checks, follow a structured diagnostic approach, and if other codes are present, check what they mean first before committing to the P0455 path.

Monitor gates and what sets the code

Before testing, understand the monitor logic. This code runs under specific cold-start and EVAP enable conditions: ignition voltage is between 11–32 V, BARO is greater than 70 kPa, fuel level is between 10–90%, EVAP purge flow is greater than 2.50%, cold startup ECT and startup IAT are within 8°C (14°F), startup ECT is less than 35°C (95°F), and startup IAT is between 4–34°C (39–93°F). The failure is based on the EVAP system not achieving or maintaining vacuum less than −2.74 kPa (−11 in H20) before the calibrated purge volume has passed. With P0455 detected, a weak vacuum follow-up test can run and passes if tank vacuum is greater than 2.74 kPa (11 in H20). That test can run for a maximum of 22 min, and on capless fuel fill systems that follow-up test and driver message are disabled.

Initial EVAP leak check

For the actual P0455 test path, disconnect the purge tube at the quick connector on the EVAP canister side of the Q12 EVAP purge solenoid valve, and install the CH 48096. Then connect the GE 41413-A to the vehicle EVAP service port adapter. Use the scan tool to command the EVAP Purge/Seal function to Not Venting so the EVAP system is sealed. Now check for leakage with the flow meter on the GE 41413-A calibrated to 0.51 mm (0.020 in). Keep the service pressure under control during EVAP work; test pressure must not exceed 13 in H20 (0.5 PSI), because too much pressure can make components behave inaccurately.

If the system leaks

If the flow check shows a leak, apply smoke to the EVAP system at the service access port adapter with the GE 41413-A. Then locate the leak with the GE 41413-SPT. Smoke testing can go faster if the system is opened opposite the smoke injection point until smoke is observed, then closed back up for testing. For intermittent leaks, move the EVAP components while watching for smoke with the GE 41413-SPT. Individual pieces can also be isolated with the J 41413-300 adapters. Once the leak is found, repair or replace the affected component as necessary.

If no leak is found, check purge-path restriction logic

If no leak is detected, the path changes from leak hunting to checking whether the purge side is restricted. Connect the GE 41413-A nitrogen/smoke hose to the J 41413-311. Disconnect the hose at the fuel cap end of the GE 41415-30 and connect that hose to the J 41413-311. Install only the filler neck end of the GE 41415-30 to the vehicle. With the engine idling, use the scan tool to command the Purge/Seal function to Not Venting, sealing the system, then command the EVAP purge solenoid valve to 30%.

Compare gauge vacuum to the tank pressure signal

Now compare the vacuum/pressure gauge on the GE 41413-A with the scan tool Fuel Tank Pressure Sensor parameter. Both should show vacuum. If the gauge shows vacuum but the Fuel Tank Pressure Sensor parameter does not, replace the B150 Fuel Tank Pressure Sensor. If neither one shows vacuum, repair the restriction in the purge path. A leak can sometimes only show up under vacuum, so using the Purge/Seal function to create vacuum and watching the Fuel Tank Pressure Sensor parameter for vacuum decay can help catch that kind of fault.

Final sensor range check and repair verification

If both the gauge and the scan tool show vacuum, continue the check. Verify that the Fuel Tank Pressure Sensor parameter increases to greater than 3.2 V, and that the pressure values are similar between the scan tool Fuel Tank Pressure Sensor and the vacuum/pressure gauge on the GE 41413-A. If it is not within the specified range, replace the B150 Fuel Tank Pressure Sensor. If it is within range, that part of the test is OK. After any repair, keep verification separate from the pinpoint testing: verify the repair and confirm the code stays gone.

Takeaway and CTA

The key on P0455 is not to jump straight to parts. Seal the EVAP system, measure for a large leak, smoke it if it leaks, and if it does not leak, prove whether the purge path and tank pressure signal agree under commanded vacuum. For more diagnostic training, visit stepdiagnostics.com.