Will ECU with slightly different part number work for my 2001 Toyota Camry?

For the most part, no, you cannot replace one ECU with another without serious modification. The ECU may bolt up and the wires may fit, but with a different part number, most likely the tune within the ECU will not work in its place. There are other things besides the physical aspect, such as the ECU having the vehicle itself programmed into it so the two work together (security aspect). If not, the ECU will reject the connect and won't start the vehicle. There's a plethora of reasons why it won't work, with very few for why it might.

To answer the underlying question of improving your fuel mileage, you need to find the reasons why it's getting poor fuel mileage in the first place:

• Look at what the manufacturer states should be the fuel efficiency for the vehicle. If you are in the ballpark, then an ECU change (or even a retune) isn't going to do much if anything for you.
• Check with other owners of vehicles of your same year/make/engine/model and see what they are getting for fuel mileage. Then compare the numbers again.
• If you're way out of the ballpark for the numbers from the above two, then consider if the vehicle may need a tune up. There are wear items, such as O2 (lambda) sensors which have a huge impact on how the vehicle runs. Spark plugs, air filter, type of fuel, etc, also play a large part in fuel economy.
• How do you drive the vehicle? If you are stop-light to stop-light pedal to the floor, consider changing your driving habits. This saves not only fuel, but wear-n-tear on your vehicle as well.
• If everything else pans out above without issues, consider buying a more fuel efficient vehicle. This is truly the easiest way to improve your situation.

These are just a few things to consider. Changing out an ECU is not going to give you what you think you want.