I have a 2012 Ford Focus that was idling rough. I brought it in to a local shop who showed me some very corroded spark plugs. They replaced the plugs and also an air filter.

When I picked up the car, things seemed fine. After driving to and from work, the engine light comes on. I bring it back to the shop, they run the codes and tell me it's an EVAP leak - completely unrelated to the work they've done. They reset it and tell me it might not come back, but it's completely safe to drive.

After driving to and from work once more, the check engine light comes on.

The shop worker tells me that it's an entirely different system and, in no way, has anything to do with their service. I don't know much of anything about cars but it the timing seems awfully suspicious.

Is there any way for me to determine if the issue is related to their work, or just bad timing?

EDIT - Small additional note, I have not gotten gas since the start of this sequence of events.

  • Is you focus a cap-less system? If so you may need to use the tool that came with the vehicle that allows you to put fuel into the vehicle using a hand held fuel tank. Looks like a small narrow funnel. Push it in and out a few times. On cap-less systems sometimes dirt gets on the flap causing a small leak.
    – cano
    Commented Jul 29, 2017 at 22:14

1 Answer 1


You can check the vacuum line for the purge valve and the related connector. If you follow the under hood vacuum diagram, it'll lead you to where the valve is. More than likely it's just a coincidence. If you've gotten gas lately, it could be a loose gas cap.

If they provided you with the DTC than it may help point to what went wrong. An unplugged purge valve connector would set a DTC immediately, an unplugged hose or loose gas cap wouldn't and would require a few trips to set. An unplugged hose may lead to running problems as well.

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