How do I reset ECM codes

now i believe as stated earlier, codes need to be manually reset. thanks for the replys

My check engine light jsut came on and when I was low on gas and when I went to fill up my gas cap was off. I take it that was probably what caused my light to come on. I might not have put the gas cap all the way on and when I went for a quick 4x4 the day before it might have loosened it all the way off with all the bouncing and stuff. How do I reset the codes manually or do I need the code reader for it? I have a friend of a friend with a code reader how do I find the specific codes for my vehicle (2001 sonoma ZR2)?

Thanks

A loose fuel cap can set a DTC for the EVAP system. Usually if you tighten it up, the SES light will eventually turn off on it's own. The DTC will remain in the history file for a certain number of drive cycles after the problem is not detected any longer, then it will be automatically deleted. The ONLY way to clear DTC's, clear the history file, and reset the monitors is with a scan tool. Some scan tools will turn off the light and reset the monitors but will not clear the history file. Code readers are just that, that's all they do and will not turn off the SES light. Disconnecting the battery will turn off the SES light in most cases, provided there are no hard faults present.