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After seeing my CEL come on in my '05 Mazda 3 (just passed 100K mi.), I used my code reader to find out that the code was P0442. So I naturally came to this website to see what I could find on the subject matter. The only question on this site (at the time of posting) about this is questioning the validity of the CEL. I personally am going to believe the CEL/code is correct, because of the data that I have on my MPG (yes, I'm nerdy enough to have kept track of my MPG from the day I started driving this car). As you can see, my MPG has been plummeting for the last few months, and given how my current tank is progressing, I doubt it is going to go back up.

Gas Mileage

I also googled around and got pretty inconsistent answers being given from gas cap replacement (my current one looks fine to me), to the neck that the gas nozzle goes into (also looks fine to me), to plenty of other theories. Anyone have any idea as to whether this is something that is as simple as a new gas cap, or something that I can do on my own or if this needs to go into the shop ASAP? Is there anything else I need to check to make a better determination?

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The OBD2 code P0442 means that on your system you have a small air/vacuum leak. Apart from the seal on the gas gap, you must check the purge valve and the fuel tank pressure sensor are both working correctly. EVAP faults can be be notoriously difficult faults to pinpoint as the system only checks itself when the vehicle is actually operating and the pressures and vacuums are very small. A Ford/Mazda scanner check on the electronics and a smoke test on the EVAP system should isolate your problem. My attention would be very much on the purge valve. IF it is leaking by not closing fully to seal the system, the engine would run on the lean side. The engine ECU would then richen the mixture and your fuel consumption would increase. Tests first.

  • First, thank you for your answer. Can you tell me where I could expect to find the purge valve and the fuel tank pressure sensor? How exactly do you run a smoke test on the EVAP System? – tarheel Aug 15 '14 at 4:33
  • flickr.com/photos/26520505@N07/favorites/with/7235952028/… There is a run through of position of the valve, and testing the valve is also included. – Allan Osborne Aug 15 '14 at 21:10
  • Thanks for the pictures, it was really easy to get into the tube that you were talking about, but I couldn't find a screwdriver that was both small enough to fit in the tube, and large enough to turn the cylindrical screw inside. Any idea of what size would accomplish that? – tarheel Aug 30 '14 at 19:15
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There a several things, that I would check at first.

  1. Gas cap or the seal on it.
  2. The connection between gas tank and gas cap.
  3. The seal between gas tank and the fuel pump under the rear seat.

Can not believe, there could be the leak somewhere else.

  • 1) To my naked eye, the gas cap looks perfectly fine, is there something that could be wrong, but not obvious at without looking in further detail? 2) Same goes for the connection between the gas cap and gas tank, is there something that I should use to test that? 3) I am still looking for this one, and seeing if I see any problem with the seal. – tarheel Aug 15 '14 at 4:37
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We have found a common failure with the canister vent valve on Mazda 3 and 5 of varying years. Last week we did 1 and this week we have 2. All with a code of P0442. This code is normally set after filling up gas and driving roughly 100 miles. The vent valve is located near the gas tank under a shield.

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I have a 2004 mazda 6 with less than 100k on it. It sits for long periods of time and the gas has alcohol in it. In my case the code could be a result of the e10 gas causing the deteriorating seals in the solenoids that hold in the vapors. This is a cheap and easy fix. Under normal conditions these seals would be fine. I only put e-free gas in my BMW and have never had this problem. My wife's Subaru has this very same issue and it is not the gas cap. Her car has very few miles for its age and we live in a hot, wet climate with crappy gas stations. Over time that bad gas wreaks ha ok on engine and fuel system seals.

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Same code, due to a very small leak in the filler neck. I found it by taking the neck off the car (easy, at least on my Cavalier), and blowing into its open end (with the cap installed). When I put the pipe in water, bubbles appeared.

OTOH, I find it hard to imagine a neck leak dropping your MPG so dramatically. Sometimes, more than one thing can be broken. When did the CEL first appear, relative to your MPG drop? Do you smell gas around the back of the car? Is it leaving spots? Have you changed your driving pattern?

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