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Consider this scenario:

  • The CEL (check-engine light) is on.
  • The fuel cap is subsequently replaced.
  • The fuel tank is half full.

I am wondering:

  1. Is it advantageous to fill the tank to the top to speed up the time it will take the CEL to go off? (we assume here that the fuel cap is the culprit)
  2. Whether the answer to (1) is yes/no, after how many miles does the CEL typically go off? Or does the air vaccuum mechanism on the fuel tank operate every so many minutes, and so it's the driving time, not distance, that matters?
  3. Once the CEL is no longer on, does the mechanic disadvantage the vehicle on the bi-annual report? In other words, is one better off reading the old codes with an OBDII reader and clearing them?
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  • I have to assume the CEL is for the gas cap?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 4, 2016 at 15:58
  • CEL=check engine light
    – Moab
    Commented May 4, 2016 at 16:13

1 Answer 1

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There is know way to know how the software was programmed for any given car, from that I know most systems will not do a fuel tank integrity vacuum check when the tank is either past 3/4 or below 1/4, so filling the tank will extend the time before it re-runs the test and turns off the CEL if the test passes. The test will only run when the tank is between 1/4 and 3/4.

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  • Makes sense. Thanks. What about the distance? Will in-town trips do it (to turn off the CEL), or is it necessary to find an excuse to go on the highway for a couple of hours? Also, do you lose points during checkups because the history will show that the CEL used to be on?
    – Calaf
    Commented May 4, 2016 at 22:43
  • @Calaf monitors are dependent on drive cycle conditions and not really mileage. in ny state for example, monitor readiness only has to do with if your car will pass it's annual inspection and if enough monitors are ready but one isn't the system just alerts the owner to this. also once the code is cleared by a scantool or the ecm turns off the mil there is no record of the dtc. however if your car is a 2011 or newer if you clear the code with a scantool in mode $11 there will be a permanent dtc that won't clear until a number of good drive cycles is reached.
    – Ben
    Commented May 4, 2016 at 23:41
  • @moab, so if your fuel level sender takes a crap the ECM will never run the test...?
    – Jimmy Fix-it
    Commented May 10, 2016 at 5:41

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