how long does it take to erase OBD codes after unplugging the battery

Question

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.

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Edit - 1

I let the engine run for a little longer and check engine light came on. OBD Scanner shows P0300/P0302(Cylinder 2 misfire)/P0304(Cylinder 4 misfire). I feel that as the car gets hot, it shows these codes. Does it mean spark plugs that I used are not able to handle the heat? Manual says use Iridium spark plugs while I used Platinum spark plugs. Is it worth getting set of Iridium spark plugs?

Answer 1

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.