Full write or partial write

The difference

A full write puts the entire memory area back into the ECU. Everything is
replaced, whether it changed or not.

A partial write puts back only the region that has changed, normally the
calibration area holding the maps, and leaves the rest of the memory alone. Tools call this
various things, including differences only or calibration only, so the name on your screen may not
be the name used here.

Partial is quicker. That is not a small point: the window in which an interruption can ruin the
job is shorter, so there is less exposure. A full write leaves the ECU in programming mode for
longer, which is exactly why voltage stability matters more on one than the other.

When a partial write is correct

When the file you are writing came from the read you took off that same ECU, and only the
calibration has been changed. That is an ordinary remap, and it is most jobs.

When a full write is correct

• Recovering an ECU after a failed write, where you cannot know what is currently in there
• Any time the file differs outside the calibration area
• After a bench or boot recovery
• Where the ECU has previously been written by a different tool or method
• Any time the tool tells you a full write is required. It is not asking.

What goes wrong if you choose wrong

A partial write, where the file actually differs outside the region being written, leaves the
ECU holding a mixture: the new calibration alongside the old everything else. The two do not
agree. What you get is a checksum error, a no start, or a car that starts and runs badly, and it
is not always obvious which of the three you are looking at until the customer has driven it.

A full write where a partial would have done is not usually harmful. It takes longer and gives
an interruption more opportunity, which is a reason for good preparation, not a reason to avoid
it.

If you are not sure, write the full file

Correctness beats speed here. A full write with a proper battery support unit on the vehicle,
a solid connection and a laptop that will not go to sleep is a routine job. A partial write into a
mismatch is a recovery. Whichever you choose, do not interrupt it once it has started, and make
sure you have the original read saved before you begin.

One rule that sits above both

The file you write must have come from the read you took off that ECU. Not a similar car, not
last week’s job on the same model, not a file with the right name in the right folder. Mixing
files between vehicles produces exactly the same symptoms as choosing the wrong write type, and
it is harder to spot afterwards.