I have been dealing with Engine Misfire codes (P0300 to P0304) on my 2003 Toyota MR2 Spyder. I changed the spark plugs and engine coils. That did not clear the codes. I used OBD scanner to erase the codes but every time I did, P0300 to P0304 kept coming back instantly and check engine light also came back instantaneously.

So I decided to go old school and unplug the battery and let the car sit overnight. I turned the car on this morning and no codes showed up and check engine light did not come up either. I Kept it running for 10-15 minutes and still no codes showed up and no check engine light came up.

I am not sure if the problem is fixed or not as I do not know if the previous attempts of erasing the codes did not erase the codes or do I need to keep the car running for a little longer for the codes to appear again.

Any help is highly appreciated.

  • Welcome to Motor Vehicle Maintenance & Repair! Jul 18, 2022 at 15:26
  • If you have a new question, please do not edit your original question ... post it up as a new question. They are related, but completely different. Jul 18, 2022 at 17:37

2 Answers 2


It is going to differ by vehicle, but I usually give it about 30 minutes clear the codes and reset the ECM. You have to let the residual charge which is built up in the electronics time to burn off. Probably takes less than 30 mins, but I don't want to have to go through it all again if I don't let it sit long enough.

If the codes were coming back instantly after clearing, but before start the car, the codes were not cleared. More than likely in your case, the ECM was in a bad state. It happens sometimes (but not often at all). By you unplugging the battery and getting it to reset, you are not only clearing codes, but also clearing the learned maps, fuel trims, etc. This could get it back to how it was from the factory. The codes may come back, but realistically with random misfires, you're going to feel the engine running differently when it does happen. If the engine feels like it is running fine (idles with no bobbling and drives with expected amount of power with no discernable misfires), I'd suggest you're golden.

  • Codes came back with same misfires. Cylinder 2 and Cylinder 4.
    – Asdfg
    Jul 18, 2022 at 16:31
  • But, that'd be a different question. I was answering your original question. If you have a new question, please post it up as a new question. Jul 18, 2022 at 17:36
  • yeah. I understand. I will post a new question.
    – Asdfg
    Jul 18, 2022 at 19:50

If your using an OBD scanner there is usually something on there for your emissions. Go to diagnostic then you wanna go into I/M readiness... if the system control says N/A that just means ur specific vehicle doesn't use that however if it says INC Meaning incomplete then there isn't enough data recorded yet. Generally speaking after you unhook your. Battery you need to allow the car to heat to normal operating temp, AND drive at highway speeds for a minimum of three mins and if u can safely allow vehicle to coast and decelerate to around 20 mph then back up to 55mph for around 5 mins your system controls should all register enough data to go to "OK". Once all are on ok your check engine light will more than likely come back on rather quickly after if the issue hasn't been fixed. Keep in mind your check engine light doesn't come on RIGHT when it happens usually. I believe and correct me if I'm wrong here but you will throw the code it will save in your banked codes and if the code is thrown a SECOND time the check engine light comes on. Hope this helps.gluck

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .