My check engine light is on. After reading the codes, I saw a few misfires. I checked the spark plugs and they needed to be replaced.

I'm not sure yet if that was the problem. But the question is do I need to reset the codes and if the problem persists the Check Engine will light up again or should I leave it as it is and it should turn off on its own if the problem has been fixed?

 

Misfire codes will not immediately go away by themselves until they're cleared. They will stay to let you know a misfire happened. There's also the possibility that the ECU has "learned" about the misfire and made "adjustments" to fuel, timing, and maybe other changes. To ensure a properly running engine, clear the codes and reset the ECU to erase any of the faulty learnings. It might eventually "un-learn" the faults and lose the codes by itself, but will probably run rough until it does.

 

One of the other errors I got is P15E8 Manufacturer control. I cannot find anywhere what that means. Any ideas?

1. Can this error code be just from a previous engine check or if OBD tool shows me the code it can only mean that it happened now?
2. Should I reset the codes in thise case?

 

According to the Bentley manual P15E8 means "Engine shutdown time, plausibility: Time too short relative to temperature drop in engine coolant." What does this mean??? My guess is something wrong with the coolant system --- maybe the temp sensor is disconnected or faulty. I'd clear the codes and take it for a short drive, or let it idle in an open garage if not registered, while monitoring coolant temp. Maybe check fan operation at the same time. Fan should cycle on about 220 deg F and off about 180 F. If the code doesn't recur, it's old news.

Strongly suggest obtaining your own Bentley service manual.

 

Well, I tried to reset the codes with my "Motopower MP69033 OBD2 Code Reader" tool that I have, but I always got "Erase Failed" message.

I decided to just start the car and see what happens. I no longer have Check Engine and error codes!!! So either the tool did clear the codes despite the error message, or changing the spark plugs fixed the problem. I think the latter is true.

Disclaimer: I had my car battery disconnected the whole time. I connected it back after changing the spark plugs and fuel filter. Can disconnecting the battery reset the error codes?