Check engine light (P0138) came and disappeared a day later.

What is your question?

Few months ago due to recall and check engine light the Toyota dealership changed the ECM control for my car. Two days ago I had the engine light back on. I checked and found the code P0138 but it disappeared a day later. Can you explain this please? Do you think it is conneced to the ECM changed control, and should I take it back to the dealer? Thanks