Trying to repair my ECU.

I have thought more about a fix. The main rework problem is soldering the tab on the sot223, due to the flex circuit bonded to enclosure heatsink. It is likely the circuit was infrared reflowed in a fixture prior to peel and stick to aluminum chassis. At solder time the thermal inertia would have been low for quick solder. Rework has two issues, getting enough heat to solder tab, yet avoid thermal damage to transistor, and adhesive that bonds flex to chassis.

A good alternative may be to just cut and remove small leads, then outboard a transistor. There are larger TO220FP package parts, that have an insulated package and also 10 times the power handling. With a single 4-40 screw, mounted nearby inside the chassis, and wired to small lead pads.

The use of the sot223 package in the original designs was risky. Just seems enough for a high impedance fuel injector, but no safety factor for a defective (shorted coil) injector. The photos in the post shows components shifted in the solder process. That may have happened if the fixture was bumped in the process, or too much solder paste was applied, floating parts. Too much solder also would increase thermal resistance, leading to early failure.