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?

6 Answers 6


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, 2014 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. Aug 15, 2014 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, 2014 at 19:15

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, 2014 at 4:37

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.


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.


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?


I just finished a faulty evap repair on a Mazda 3. I'd like to share the experience.

  • 2008 Model Year, Mazda3, manual, 2.3L engine.
  • Check engine lamp, read codes P0442 (Evaporative Emission Small Leak) and P0455 (Evap Emission Large Leak)
  • Read up on line. Most likely source is fuel cap, followed by engine compartment mounted purge valve.
  • Test purge valve, works correctly, no leak. Reinstall old purge valve.
  • Purchase new gas cap, clear stored codes.
  • Check engine light comes on again with same codes.
  • More research reveals potential trouble with fuel delivery module lock ring cracking. Drop the fuel tank 1", use inspection mirror and flashlight to inspect the fuel ring. I was only able to inspect 180 degrees of the plastic lock ring, but it looked just fine. Assume ring is good.
  • Build an awesome smoke detector to troubleshoot the car. Smoke injected to vehicle vapor recycle port at the engine compartment at 0.5 PSI.
  • Use smoke to verify the rear carbon canister breather valve is working correctly. That valve is normally open. When set to closed, there is no smoke leak.
  • At this point I can verify that no smoke is coming out anywhere. I can hear the rush of air into the system, but no smoke is escaping. I'm presuming that the port I'm using is on the other side of the carbon canister, and the canister is filtering the smoke out before it gets to the fuel tank. I can smell gasoline from under the car when this is happening. How is this possible?
  • Empty the fuel tank. (Don't skip this step; this makes the job WAY easier...)
  • Remove fuel tank from car, carefully inspect top of tank. Sure enough, the fuel pump module lock ring is split, in exactly the location I was unable to inspect thru the access hatch. Ugh.
  • Replace the lockring. Retest vehicle.
  • I will say removing that tank was one of the most UNfun repair jobs I've worked on lately. Getting those breather hoses disconnected was a total pain in the neck.
  • Perform the emission tests EXACTLY as dictated by the factory service manual. This was very time consuming.

Obviously I would have saved a lot of time and some money had I done a better job of lowering the tank and inspecting the lock ring the first time. I initially had lowered the tank by nearly removing just the front mounting bolt. Had I loosened all three mounting bolts I suspect I would have had enough room to inspect the entire circumference of the lock ring.

I'm hoping these notes help others during troubleshooting and repair....

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