What this code means
P0496 means the ECM may be detecting EVAP purge flow or vacuum when purge should not be occurring.
What the vehicle may do
- The vehicle may run normally while the EVAP fault is stored.
- The malfunction indicator may come on after the monitor fails.
- Fuel tank pressure readings may be affected by vehicle temperature, shop temperature, fuel level, and vapor pressure.
Possible fault areas
- Possible purge control issue
- Possible EVAP purge solenoid valve installation, damage, blockage, or restriction issue
- Possible command-versus-fuel-tank-pressure-response issue
Diagnostic path
Opening context
On this 2014 to 2018 GMC Sierra 1500 with the 5.3 gas engine, P0496 means the ECM may be seeing EVAP flow when it has commanded a non-purge condition. In plain language, the tank can be pulled into vacuum when the system should be sealed off from intake manifold vacuum. The truck may run normally, but it can also show an EVAP fault, and the possible fault area is often around purge control, purge valve condition or installation, a temporary restriction, or the way the fuel tank pressure signal responds during the test. Start with the basic system checks, then follow a structured diagnostic approach before getting into the P0496-specific test.
Understand the monitor
The logic here is command versus response. The ECM seals the EVAP system by commanding the purge solenoid OFF and the vent solenoid ON, then watches the fuel tank pressure sensor to see if vacuum is being drawn into the EVAP system when purge should not be happening. Treat the run criteria as monitor gates: ignition voltage between 11–32 V, engine OFF time greater than 48 min, BARO greater than 70 kPa, fuel level between 10–90%, startup ECT less than 35°C (95°F), and startup IAT between 4–34°C (39–93°F). P0496 runs once per cold start, for up to 17 min, with the purge valve and vent valve closed. The code sets when the ECM detects greater than 10 in H2O vacuum for 5 s during a non-purge condition. It is handled as a Type B DTC for setting and clearing behavior.
Reproduce and verify before testing
Next, reproduce the operating conditions for the code. If the captured conditions are available, use those conditions as your guide. Then verify whether P0496 resets. If it does reset, continue into the P0496 testing path. If it does not reset, the result is all OK at that time. Keep in mind that vehicle temperature, shop temperature, fuel level, and vapor pressure can affect the fuel tank pressure sensor reading, so let the vehicle temperature stabilize to the shop temperature for the most accurate check.
P0496 testing path
For the focused test, turn the ignition ON. With a scan tool, command the EVAP Purge/Seal function to Not Venting. Make sure the EVAP Purge Solenoid Valve command is 0%, or this test will not work. Then run the engine and watch the Fuel Tank Pressure Sensor parameter. The expected result is less than 1.9 V for greater than 90 s.
Test decision
Use the fuel tank pressure reading to make the decision. If the parameter is greater than 1.9 V, the specified outcome is to replace the Q12 Evaporative Emission Purge Solenoid Valve. If the parameter is less than 1.9 V, the result is all OK. If the path does not line up cleanly with what the truck is doing, pause, recheck the earlier diagnostic path, and avoid guessing.
Repair verification and takeaway
After the repair is complete, verify the repair and confirm the code stays gone. The takeaway on P0496 is simple: the ECM is looking for vacuum during a non-purge event, so keep the diagnosis centered on the purge command, the sealed EVAP response, and the fuel tank pressure sensor reading. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0496 is often diagnosed by comparing the commanded non-purge state against the fuel tank pressure sensor response.
For more guided automotive diagnostics, visit STEP Diagnostics.
