What is Car ECU reflash?
The Engine Control Unit (also called ECM Engine Control Module) is a digital component embedded in latest vehicles running a program that hold, reads and control things like key data, 02 and CO2 sensor output, engine rpm, engine performance, fuel to air ratio, timing variable valve and more.
The ECU is basically an EEPROM (Electronically Erasable Programmable Read Only Memory) computer chip which means the chip retains its data without periodic electronic power; loss of electric power will not cause the chip to lose its programming data.
When losing the master transponder key on some vehicle (Lexus/Toyota) there is no interface of making a new key, but reflashing or replacing the ECU. Also if the ignition or the ECU is being replaced on a vehicle a reflash service would be necessary.
Reflashing the ECU actually involves electronically deletion of the read only program and download a new one updated map information instead. Typically, when an ECM is re-flashed, a new map is assembled that allows the engine to run better and faster, but can decrease its durability and reliability.