Can fault code manually be rest?

It's not recommended but disconnecting the battery will likely clear it. It will also clear a lot of other things that you would rather not clear.

How do you know the sensor was the problem? What was the code number?

Dear Sir,
I knew from the mechanic which use OBDII scanned. I have new information. Yesterday I had cleared the code by disconnecting the battery as you and the mechanic told me. During driving home about 20 KM there was no CEL light up. But this morning I drove the same route but CEL light up after about 10 KM.
And I am not sure the simple method to check the fault code by turning the ignition switch to running position but not to start and count for the set of CEL (fault code), in my case there are 6 counts it means something wrong about the lambda sensor! There are many things which relative to the faults of the lambda sensor. In case I still not going to mechanic shop, what is the things should be done at first step and next
Thank you very much,
nattapong c

I suspected the sensor wasn't the actual problem. They rarely are. Just because a code refers to a sensor doesn't mean the sensor is to blame. We need the exact code number starting with a "P" to be able to help you.

Dear Sir,
That mean I need scanning to get the exact code to know the problem information.
Thank you,
nattapong c.