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O2 Sensor Error Code vs Loose Gas Cap

Occasionally, my gas cap is screwed on tight enough. The check engine light comes on, usually when the gas tank is just above half. I unscrew it and screw it back on. Eight to ten engine starts later, the check engine light goes out. This time the light was on during a maintenance stop at the local repair shop. The "computer" says O2 sensor malfunctioning and I need to replace it. Gas mileage is about the same as usual. Car sounds fine. The shop says the code indicates an intermittent problem and needs replacing. It was replaced in 2008. I think it's just the gas cap. Thoughts? 2004 Vibe, about 95000 miles.


  • edited February 2010
    Hello, next time you have the car scan, just ask for the codes, it will help you to know what the on-board computer is seeing as a fault. You should have the 4 wire Ox sensor and the replacement interval is usually 100k miles. Good luck.
  • edited February 2010
    The codes for loose gas cap and O2 sensor problems are different and there is no interaction between the two. O2 sensors do get tired and 95000 miles is definitely in the range where failure is plausible.

    Most cars have two O2 sensors. It's likely that the one replaced in 2008 is not the one that is giving you trouble now. The O2 sensors are used to fine tune the fuel air mixture. If one or both fail, the car will simply use a default fuel mixture and will probably run fine albeit with slightly greater emissions and perhaps slightly lower mileage.

    OTOH, a few vehicles simply fire off O2 sensor errors at random. Our 1995 Dodge Neon reported one every six months or so. I simply cleared them and only replaced the sensor when the error persisted in coming back. But the Neon allowed me to read out a simplified version of the codes without a scan tool and to clear the codes by pulling a fuse. I don't think your Vibe has the capability to read codes without a scan tool, and I have no idea which fuse -- if any -- will clear the stored errors.

    If you aren't going to replace the O2 sensor immediately, I'd suggest buying a cheap scan tool (about $60-$80) at Sears or Autozone that will allow you to find out what is turning the check engine light on, and to clear the codes. But there is actually a problem and you haven't fixed it, the codes will likely come right back.
  • edited February 2010
    We say it over and over: there is NO trouble code which says a particular part is bad and should be replaced. None. Even some "mechanics" make that mistake.
    The trouble code indicates that there is a problem in a particular CIRCUIT. There is a component (a sensor, or a control valve) IN that circuit; but, that component is only PART of the circuit. So, the entire circuit needs to be troubleshot.
    Now, an oxygen sensor CIRCUIT needs to be troubleshoot to find out what caused the trouble code to come.
    Bring the trouble code to us for advice. It will be something like: P0310. If you need the code scanned, you can have that done (for free) at an auto parts store. Ask them.
    When the car is scanned, copy down the flashing icons (if any) that show on the scan tool, and bring those here, also.
  • edited February 2010
    Sara - there is no code that says that an O2 sensor is malfunctioning. It just doesn't work that way. There are lots of codes that get people to say "probably an O2 sensor" b/c those sensors are part of a whole system of things going on. The problem can be elsewhere in the system. The landfills are full of perfectly good parts that were replaced based on codes without diagnosis.

    As noted - get the actual codes (format P0123) and post them here.
  • edited February 2010
    I had a similar problem with my 02 Impala and always thought I was just consistently putting the gas cap on wrong...when I brought to my mechanic they replaced one of the O2 sensors and the check engine light has not come on since.
  • edited February 2010
    I think it's just the gas cap.

    What facts do you have to back up your conclusion?

    The O2 sensor is part of the fuel delivery feedback loop.
    The gas cap is part of the evaporative emissions control circuit.
    These are completely independant systems and the control unit has separate error codes for those systems. IMO, the gas cap causing an O2 sensor error is about as likely as the hub cap. So, I'm interested in your logic...
  • edited February 2010
    My logic isn't very logically. Basically, the check engine light comes on. I go to the gas cap, take it off, put it back on, count the number of times I start the engine. The light goes out after 8-10 engine starts and doesn't come back on for 6 - 12 months.

    Where it gets confusing for me is if the the O2 sensor fails intermittently, why so infrequently or why does the error code go away by itself?

    The responses to this request has given me enough background to ask intelligent questions. I didn't realize there were 2 O2 sensors for example.

  • edited February 2010
    Unfortunately car makers and their associated dealership service departments have played a role in keeping people uninformed about the "dreaded" check engine light. Owner's manuals say almost nothing except tighten your gas cap and then take it to the dealer if that doesn't work.

    Take 5 minutes and peruse this:

    It is a list of "diagnostic trouble codes" (DTCs) and one or more of these is stored in the computer when the check engine light is set. Not all of these are as common as others, but as you can see there is no simple way to deal with it if one doesn't have the actual code.

    Each code, btw, has a set of associated diagnostic steps that one should follow to pinpoint the issue. A leaky gas cap would likely produce something in the P044x range. The "its an O2 sensor" assumption can come from a large number of codes b/c the O2 sensor's job is to measure stuff in your exhaust and report to the computer. Sometimes people think an O2 sensor is bad b/c it is doing its job (shooting the messenger). Of course, O2 sensors do go bad but its really frustrating to people who get them replaced to find the annoying little light still there.

    Just FYI: if that light ever starts flashing at you don't continue to drive until the issue is sorted out.
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