Ground rules: never flash a ROM you can't account for — you built it from YOUR stock read, or a professional tuner built it for YOUR car and CAL ID. Random forum ROMs for a 'similar' car are how engines and ECUs die. And never flash with a marginal battery.
Step 1 — Pre-flight: battery on a charger/maintainer at a healthy voltage, key ON engine OFF, every electrical consumer off, laptop plugged into wall power with sleep disabled, Openport direct into the laptop, cable firmly seated. Read the current ROM one more time first if you have any doubt about what's on the car.
Step 2 — Open your modified ROM in ECUFlash and sanity-check it: correct CAL ID family for your ECU, and file size matching your stock read. ECUFlash validates checksums on Subaru ROMs as part of the process — a checksum complaint at this stage means stop and figure out why, not click through.
Step 3 — Start the write and become a statue. The ECU is erased and rewritten in blocks over several minutes. The single rule: NOTHING touches the car, key, cable, or laptop until ECUFlash declares success. Mid-write interruption leaves the ECU with half a program.
Step 4 — On success: key OFF, wait ten seconds, key ON, and confirm the car's normal chimes and dash behavior. Start the engine and let it idle. Expect the idle to hunt briefly on some cars while the ECU relearns — settling within a minute is normal.
Step 5 — First ten minutes: no boost. Gentle driving while you watch coolant temp and listen. If you flashed a tune with changed targets, your next stop is the logging guide — a documented pull, reviewed, before any enthusiastic driving.
Step 6 — Log the flash in your notes file: date, ROM filename, what changed, and odometer. Six months from now, 'what map is on the car?' should take ten seconds to answer, not an evening of guessing.
If the write fails or the car won't start afterward: don't panic-flash repeatedly. Go straight to the recovery guide in this section — EJ ECUs are very hard to permanently brick, and the recovery procedure exists exactly for this.
Tuning changes engine behavior. Numbers here are typical, not guaranteed — your fuel, climate, and hardware matter, and a professional calibration for your specific car always beats a generic map.