There was a thread on this a while back, but didn't find it. I have a Horizon electronic tach in an airplane with an O-360, which recently started flashing a red light at me intermittently. A study of the POH for the tach didnt' reveal what this signal meant.
In conjunction with this, started getting weak starts, as in just barely catching, and running grudgingly.
Talked to the mechanics about the issue, and couldn't come up with answers. Mag checks were normal.
Took the mags apart, and the left one was total junk inside, as in pieces of metal rattling around inside. It was still firing, but the mechanics said they don't understand how.
That was the mag the electronic tach was flashing a red light on. So, maybe the tach is better than I thought.
And I liked it anyway. For what it's worth,