99' outback check engine light

Hi, My 1999 check engine light keeps coming on.When my mechanic puts it on the scope it reads misfires in all cylinders. I don’t feel any loss of power at the time. We have replaced the coil,plugs and wires.This last time it went back on after driving for less than ten miles. Any ideas? Thank you.

If things seem to be firing ok you may want to consider replacing the knock sensor. They can be too sensitive at times and cause a problem like this.