Will disconnecting my car battery reset the on-board memory or do I have to manually erase it to remove error codes?

I ask because I have a faulty electrical system that throws gas leak errors and I cannot pass a smog test without resetting the memory. I disconnected the battery last night and drove the car after a cold start in the morning. There were multiple error codes. I don't know if these are newly generated error codes or old error codes.

Is it possible the computer memory has an independent power source or firmware like memory?

My question is, should I buy a diagnostic tool and manually reset the memory to pass the smog test?


Should it have been enough that I disconnected the battery?

My car is a Saturn Ion 2003.

  • Thanks for the question. Can you provide make/model/year of your vehicle. Some ECU's behave differently than others. Commented Dec 2, 2016 at 8:01
  • Saturn Ion 2003 thx
    – the_prole
    Commented Dec 2, 2016 at 8:37

2 Answers 2


No you can not reset errors memory by disconnecting the battery.

You need to use a diagnosis tool to reset the memory.

Some cars have also a hidden menu which can be activated using odeometer button, and you can reset the errors with that menus

Remeber to buy a proper diagnosis tool since some just reset the engine and transmission errors(obd2 elm327 compliant) but some can interact with other computers in your car and have them reseted as well.

Do a complete research and find a good one, it needs a comprehensive research over the reviews in related forums, I personally bought an interface that didn't work and many people has been alerting to not buying such a cables, but I did not search the internet to read peoples opponion about them, do not trust the reviews on ebays and amazon, just forums and oponions from old members not the new fake ones.

And remember, car computers usually does diagnosis periodically, so if you just reset the error memory without fixing the issue, expect that errors to come back again in two or three days normally, Cause the modern cars do this once in each certain period, some do the test when you disconnect and connect the battery again. So you will not have a lot of time to pass the test.

However I strongly recommend you to fix the issue and have the error memory reseted afterward. Most of the times you need a diagnosis tools to reset the error you just fixed its issue, otherwise it will stay! The car computers usually just add the errors and not erasing them automatically when it is getting fixed, though it also depends on the severity of the error.

For example : A light error(i.e left rear indicator bulb not working) maybe disapears after changing the bulb but an airbag fault will not be gone after fixing the airbag.

  • Hidden menu via the odometer button? Intriguing. Can you provide a link with more details?
    – bishop
    Commented Dec 3, 2016 at 2:44
  • @bishop there is some video on the youtube that explains how, odometer does reset the service light for sure and in some cars it can reset warnings too, Unfortunately I know a link for a special BMW model ( not all BMW and sadly not mine) but here is the link, dig more, worth to check. m.youtube.com/watch?v=EkMasUal5Vo
    – Iman Nia
    Commented Dec 4, 2016 at 23:36

Yes, disconnect both the pos and neg cables and touch them together for 5 seconds. This will clear/reset it. There's also a way to reset the ECU on BMW e46 with the key and gas peddle. Something like turn the key on but not start, press the gas peddle for 5 seconds, turn key off, remove foot from peddle. Then time exactly 2 minutes and remove the key. Insert key, start and drive. I would suggest you Google this in case I missed something. That's what I can remember from when I did it a while back. Good luck.

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