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This happened to a friend of mine: The "check engine" light came on in his truck (Toyota). He replaced the gas cap and the light shut off.

Then I also had a "check engine" light come on in my 1994 Chevy. I took it to the dealer, they hooked up to the computer and ran a full check. The service writer (not the mechanic) told me a bunch of mumblety that amounted to "lots of things could cause the light to come on, but we can't figure out which one in this case, 'cause the computer is too old." Then he told me I should get a new gas cap.

Huh? Why would that make a difference? And why can't they tell?

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There are two possible explanations:

  1. If the fuel cap does not form a good enough seal, the fuel injection system may experience problems with drawing fuel from the tank.
  2. On newer cars many of them have a fuel cap sensor to detect if the cap is not screwed in. This is related to emissions, although i am uncertain how. I know when I get my yearly emissions check the tech has to hook the fuel cap up to a machine and it tests it (I believe for pressure leaks).

My 2006 VW Golf has a specific fuel cap light on the dashboard that will come on to alert me if the cap is not screwed in correctly rather than light up my Check Engine Light.

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The EPA regulations require that the fuel tank is a sealed system so that no vapors escape. There is an entire system (Evaporative Purge) dedicated to that task. EPA regulations also require that the ECM (Engine Control Module) check the system for leaks. When the right conditions are met, IE fuel level between 1/3 and 1/2 tank, outside temp 50 - 90, etc., the ECM pulls a vacuum on the evap system and makes sure it holds a vacuum. This tells the ECM if there is a leak, allowing vapors to leak into the atmosphere. If it fails this check x number of times it turns the MIL (malfunction indicator lamp) on. This could be caused by a leaking seal, a torn hose, or a missing gas cap. If the problem is corrected the light goes out the next time the computer runs the test and it passes.

When this system first came out, salesmen and service advisers everywhere were telling anyone with a MIL light on "just tighten your gas cap the light will reset itself"

I suspect that the service adviser you were talking to (in your second question) did not know what he was talking about. If your car has a "check engine light" it has a computer and it stores a trouble code that it turned the light on for. It's crazy to say the computer is too old for us to know why the light is on.

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    1994 was before the OBDII standard, so it is possible that the mechanic was explaining that the codes were not specific enough because of the age of the vehicle.
    – jzd
    May 4, 2011 at 23:37
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    "Too old" could just mean they don't have the right OBD scanner. My last car was "OBD1.5", and no one could read it.
    – endolith
    Dec 28, 2011 at 5:02
  • Depending on the car, there is perhaps a secondary way to retrieve the codes, especially for older cars. I drove a '90 Dodge Caravan for years, nobody had the diagnostic computer anymore, even the dealership, but you could pull the codes by cycling the key on/off 5 times then count the flashed of the CEL light, look it up in a table in the service manual, etc.
    – nexus_2006
    Apr 27, 2016 at 22:11

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