I am currently developing an application which retrieves diagnostic trouble codes from my car's OBD system and display any DTCs in a list.

However, as my car has no trouble codes I am unable to test the functionality of this application. I am using mode 03 of the OBD-II standard SAE J1979 to retrieve the trouble codes using AT commands, an ELM327 Bluetooth Dongle and a Nexus 7 (2012) tablet. While I know that this functionality works in theory, I would also like to test that it works in practice, so that the application can be developed further down the line.

I was reading that the 'Check Engine' light on my car could be activated by leaving the fuel cap off the gas tank - solely for testing purposes. My car is a 2005 Ford Fiesta Mk6.

My question is whether doing so could cause any damage to the fuel pump due to reduction in pressure for its required operation and if so, would testing of this kind be worth pursing?

Many Thanks,


  • Have you also written the functionality to read the "live" data streams - very handy and an easy way to test what is happening.
    – Solar Mike
    Commented Feb 6, 2018 at 12:19
  • This question has been asked previously. Let me see if I can dig it up
    – Zaid
    Commented Feb 6, 2018 at 12:32
  • Found it: mechanics.stackexchange.com/q/26834/675
    – Zaid
    Commented Feb 6, 2018 at 12:34

3 Answers 3


tl dr: No, leaving the gas cap off for a short time to allow the code to pop (and the CEL) will not damage the fuel system or the car itself.

People leave the gas cap off every day of the week and the CEL sets. The EVAP system is checking to ensure it is sealed, either though pressure or vacuum (depending on the system). When it detects the pressure or vacuum is not present, it sets the code because it believes there is a leak detected (and it would be right! There's a huge hole where the cap should go!). After you put the cap back on and it's sealed again, it can again get the vacuum or pressure reading it's looking for and everything is back to normal. You don't want to run around with it open for too long, but for the short time of your testing period there should be no issue or damage to the system.


Could you not just unplug the o2 sensor. That is guranteed to throw a code I think, Or at least it does on my bike. I don't see how leaving the fuel cap off would throw a code to the ecm.
Also could you not get an existing app and decompile it? I'd ask what language your doing it in but that would bring this off topic so I won't.

  • 1
    It would set a code for an evap leak once the monitor ran. O2 or something like the maf map or an injector would set a code right away.
    – Ben
    Commented Feb 6, 2018 at 12:34

If you're serious with working on an OBD2 reader, get yourself an OBD2 simulator, i.e. an actual hardware device where you can plug a dongle into. It's very convenient to develop this way.

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