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Subaru ECU bootstrap mode (boot mode) repair

There are some freely available applications which can read and write to Subaru ECUs

I recently repaired a Subaru ECU which had been locked by an ECUTek remap
As part of the ECUTek remap, they alter the stock authentication method which blocks access from any of the usual tools
Only an ECUTek tool will be able to read or write to the ECU over a diagnostic cable
It is not even possible to flash the ECU back to a stock file

As with many CPUs, it is possible to do a very low level erase and write of the data
There are a number of example DIY circuits that people have made to accomplish this, but they all looked quite complicated
All that is really needed is a 5v TTL level UART serial connection, a timed signal to keep the watchdog timer happy to prevent it from constantly resetting the CPU, and to hold several ECU pins in certain states
I managed both of these points by using an Arduino Pro Micro (5v) and an FTDI USB to 5v TTL UART cable

FTDI cable
Yellow to test pad p411 (rx)
Orange to test pad p409 (tx)
Red 5v to arduino raw (5v)
Ground to arduino ground

Arduino Pro Micro 5v
Solder the Arduino jumper to skip voltage reg
Pin 8 - p405/FWE - set high in Arduino code
Pin 9 - p413/MD1 - set low in Arduino code
Pin 10 to p407 (watchdog timer) - toggle high and low at approx 125hz in arduino code
Ground to ground on header pins, there are 2 to use, or go to C446 negative side

There are pin headers on the ECU board near the CPU but they are very small, the test pads are a lot easier to solder
This was enough for the newest version of ECU Flash to write a stock file to the ECU in bootstrap mode

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