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I lost the gas cap for my 2008 BMW 328i about a month ago. It took me about a week to get it replaced. During that time, the check engine light for my car lit up. I went out of town right after putting on the replacement cap and did not drive my car for 3 weeks.

I returned from the trip yesterday and took my car to do an emissions test today. The results came back with an "Evap Emission Control Sys Leak Detected (Small Leak)" message. I did a quick search online and it seems a missing gas cap could be the root cause here.

The emissions guy said I needed to take the car into the shop to get it inspected and figure out where the leak is. Do I need to do this if the root cause is the missing gas cap? I've hardly driven the car (under 30 miles) since I replaced the missing gas cap and before the emissions test. Would the detected leak go away if I put more miles on the car with the gas cap now in place?

For reference, this emissions test was done in the state of Georgia.

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The ECU (Engine Control Unit, ie. computer) runs a bunch of tests, and stores any errors it encounters. Because an error was detected and the ECU has not been reset, the emission testing device read the error code which was still stored in the ECU.

You should

  • Reset the computer by clearing it with an ODB2 reader, disconnecting the battery, or pulling the fuse.
  • Complete a drive cycle during which the computer can run all its tests.
  • Take the car back to the emissions guy, at which time the car should pass emissions, unless there is actually a leak in the system.

To complete a drive cycle, you can either manually perform a bunch of steps during a short period, or just drive for 100+ miles, during which time normal driving should produce all the steps.

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    While a 100+ mile drive should complete the drive cycle, it won't necessarily do it for you. One of the things which clears the EVAP stuff is ensuring the fuel tank is between 1/4 and 3/4 full ... if it isn't, it won't clear.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 27, 2019 at 21:48
  • Absolutely correct - better to research the exact parameters and requirements of the Drive Cycle for your vehicle, and attempt to perform those rather than just driving randomly. BMWs can be especially finicky. The outside air temperature has to be in a certain range, the intake air and coolant temperatures must be close (it has to be a cold start), there needs to be a long part-throttle cruise followed by a coast-down, etc.
    – SteveRacer
    Aug 28, 2019 at 3:03

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