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Q: Code P0496.

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I have a 2010 GMC Acadia. I'm getting the code p0496. I have replaced the fuel rail sensor, purge valve and the gas tank pressure sensor. My engine light is still on. What else could it be?

My car has 148000 miles.
My car has an automatic transmission.

A: This may be obvious, but have you cleared t...

This may be obvious, but have you cleared the code with the scanner? If so, disregard that first sentence. Codes will not clear immediately after the repair. With an emissions system code, there is also a drive cycle to be considered as well. The code can be cleared but the drive cycle tests must pass based on the computer system self tests. So the code will clear after a repair if the system is fixed, but it will take a few drive cycles that meet some exacting circumstances to clear the code. If the code is cleared and the light comes back on, then the repair was not successful. Again, this paragraph maybe redundant, but not everyone knows all this.

Moving on to what you can do next. There are two solenoids, so you will need to make sure you have replaced the correct one. There is a canister purge solenoid and a canister vent solenoid. This codes states that one of these two solenoids is open. Either due to an electrical problem or a mechanical problem. The PCM (powertrain control module) routes vacuum from the motor to the fuel tank and puts the fuel tank into a low pressure condition. This is done in various different ways depending on what the PCM is testing. This is a part of a drive cycle and drive cycle test happen only under specific circumstances. But we are focused on both solenoids being closed as this code indicates the PCM was unable to stop the vacuum from being applied to the fuel tank.

The best way to test this system is with a scanner and many times a smoke machine. Also needed will be the ability to get under the car and information as to how the emissions system is programmed. This will include wiring diagrams. Without this, all of us are guessing as to why the PCM is still seeing a low pressure AKA vacuum in the fuel tank. The first components to suspect, one of which you have already replaced are the vent and purge solenoids. Beyond them, you will be chasing a bad electrical connection that are typical in all systems. In this case I would be inspecting all connectors and harnesses involved in the evap system.

If you require help with this system, I recommend the following inspection to help you out. Check engine light

Good luck!

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