Toyota Echo is running great. Engine light came on. Disconnected the negative battery
And reset. Light stays off for three weeks. But now its on again??

When engine light comes on in cars. Does it have something to do with oil
Pressure and the pump. Years ago we had oil pressure gauges on cars

Your check engine light may come on for a variety of reasons - some requiring immediate attention to the engine. I would not delay in checking out the reason the light keeps coming on.

You need to have a diagnostic scan to determine what codes have been set in the computer.

Disconnecting your battery will not erase these codes, as your vehicle has an OBD-II computer system. The only way to reset the codes is with a scanner; however, if you don't fix the problem prior to resetting the light then the code will just keep being set and the light will keep coming on.

As for the oil pump, there should be a separate oil indicator light on the vehicle; this will illuminate when there is an issue with the oil pressure.

The most common CEL concern for your model Echo is P0171 system running too lean.
Do you notice a little sluggish in the morning starts and take off's?
It may be not that bad yet, but it gradually gets worse. The mass air flow sensor will need replacing when it does.
Again, this is the most common. It could be something totally different.

Can't rate this8384 answer, but agree with all the above.