I was surprised that an engine light that comes on. Could come from many
Sources. Fuel filter, oxygen sensor, cracked gas cap. It all depends on what
The scanned code is.

My Toyota echo 2003 runs OK. Occasionally the engine light comes on.
I have reset it. By disconnecting the battery cable. The light will stay off for about 3 weeks.
Then come on again?

Something is clearly wrong. It may be something small - check the manual for that.
But it may be something important also. Yes you can switch it off for some weeks by resetting it by removing the power to the central computer. But as the light comes afterwards back on, it is better to do some checking before doing the same again. It could be something very important, or an indication that something has to be replaced (for instance the camshaft belt) or a warning (for instance for a gasket leak).

Success

I was surprised that an engine light that comes on. could come from many
sources. fuel filter, oxygen sensor, cracked gas cap. it all depends on what
the scanned code is.

my Toyota echo 2003 runs OK. occasionally the engine light comes on.
I have reset it. by disconnecting the battery cable. the light will stay off for about 3 weeks.
then come on again?
With emission controls being as sensative as they are, the light will come on if a system leak is as small as the period at the end of this sentence.

If you haven't any other symptoms other than the light, it may be not a major repair. Your car may feel sluggish or lack power, indicating the mass air flow sensor has gotten contaminated and will need replaced.
Or the fuel fill cap seal not be sealing. The seal does get stiff over time and could need replaced.
Have a diagnostic scan tool hooked up to see what code or codes are stored and let us know.

Why the second post on the same subject? I already answered last night and now you're getting the same responses all over? Don't get this forum sometimes?? 9253

Try cleaning your mass airflow sensor