Is the ECU the computer? Why do you think it is worth hanging onto it?
It works like this...
Mechanic gets the problem you give him. He goes over to his computer, he puts the problem into it.
Computer says change the ECU (computer)
Mechanic changes the ECU, Everything works OK.
You get billed for a new ECU.
2 months down the line, same problem.
Mechanic gets the problem you give him. He goes over to the computer, he puts the problem into it.
Computer says change a sensor.
Mechanic changes the sensor, it does not change anything, back to the computer.
computer says change a different sensor.
Change that sensor. Works OK.
You get billed for two new sensors.
Everything from then on is fine and your happy (ish). What really happened is that you had a bad sensor, the sensor only tripped in certain situations so it took a while for it to happen again. The mechanic changed a perfectly good ECU and a perfectly good sensor before he actually hit the jackpot.
It is almost always, bad earth, bad connection or bad sensor. An ECU is a computer, it rarely has a reason to go wrong as it is encased. In spite of this because of the fact earth, connection and sensor problems are the sort of problems that come and go, on top of that an ECU tries to learn about the vehicle for the first few thousand miles, and as such often ignores bad sensors or works around them before it decides their is a problem.
keep the old ECU, if it goes wrong in the future you can always try it out.