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So recently my 07 baseline sky has been acting crappy. First off it will long crank and sometimes not even start at all until 2 or 3 tries. Then when it finally does start up the RPMs will run at like 200-500 and die if i dont rev it up. The codes thrown are P0496 which what i can find seems to be the pressure valve not opening or closing when it should or the sensor messing up. How can I best diagnos which part exactly is bad? Also I just replaced my O2 sensor on Bank 1 sensor 2 and am now having code P0420 for catalyst bank 1 sensor 1 as well.
 

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That fault code indicates that there is vacuum in the fuel tank when there shouldn't be. Since fuel tank vacuum will reduce fuel flow and cause the engine to run poorly or die, it seems likely that it is a valve that is bad, and not the sensor. There are two valves involved, the purge valve that introduces vacuum to the system, and the vent valve that opens the system to atmosphere. The code is set if there is vacuum present when the purge valve is closed and the vent vave is open, so the most likely cause would seem to be the purge valve not closing.

Why did you replace the downstream O2 sensor? If the sensor is good, and the engine is running properly, a likely cause of this code is a bad catalytic converter, but you need to fix the P0496 code first, as a poorly running engine can cause the P0420 code.
 

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Discussion Starter · #3 ·
That fault code indicates that there is vacuum in the fuel tank when there shouldn't be. Since fuel tank vacuum will reduce fuel flow and cause the engine to run poorly or die, it seems likely that it is a valve that is bad, and not the sensor. There are two valves involved, the purge valve that introduces vacuum to the system, and the vent valve that opens the system to atmosphere. The code is set if there is vacuum present when the purge valve is closed and the vent vave is open, so the most likely cause would seem to be the purge valve not closing.

Why did you replace the downstream O2 sensor? If the sensor is good, and the engine is running properly, a likely cause of this code is a bad catalytic converter, but you need to fix the P0496 code first, as a poorly running engine can cause the P0420 code.
Thanks for the prompt reply. That makes sense what you have said. I know there are 2 valves, one on top the manifold and one back by the gas tank itself. Are you suggesting that the faulty valves is the one on the manifold?
So I replaced the downstream sensor because originally the code for that was being thrown about a month ago. Sure enough I checked the readings of it and the numbers were constantly fluctuating way over the threshold so I knew it was a faulty sensor. I checked back pressure on the front side of the cat itself and there is no clogging so it must just be my front sensor is bad(it was indicating a bit above normal readings too on the diagnostic pit). I have been wondering about the P0420 being a response to the P0496 myself. Question is which valve needs replacing?
 

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The valve on the manifold seems the most likely to me.
 

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thanks for the notice, how would I go about diagnosing whether it needs to be replaced or not?
The valve is normally closed, so i would unplug it with the engine running to make sure that it is off, then pull the vacuum hose off of it. If vacuum is present at the port, the valve is bad.
 
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