I’ve never seen this on a modern Stern, even though I’ve had plenty of EOS and actuator issues.

Do you have the right length coil sleeves? If the sleeve is too long and it prevents the plunger from going all the way in, this could happen as the plunger linkage is getting choked by the end of the sleeve. At least, this happened to me recently on my Old Chicago and I imagine could happen on a modern Stern as well.

IRL540N rings a bell to me more than that other code. If nothing has changed aside from the transistors, then it does seem pretty likely that’s the issue...

Try adjusting your EOS so that the gap is wider when the flipper is fully extended. Those look like they could be too close...a tiny bump on the flipper might be closing the switch, which causes it to re-engage.

After trying that adjustment...

Another thought...is it possible you have the wrong coil stops? When the flipper is engaged, the plunger should be making contact with the coil stop on the other end. I'm wondering if the coil stop is too short....maybe the plunger can't go all the way, so it's in a state of hovering/suspension that re-engages when slight pressure is put onto the flipper?

Okay, if the plunger is hitting the stop before the linkage hits the coil, then it's probably not the stop.

Did you adjust the EOS switches like I suggested?

Good suggestion. Make sure all the switches register correctly in switch test mode and there are no switches that are unexpectedly triggering other switches. Pound around on the playfield to make sure vibrations aren't triggering anything.

Still, it sounds like it's not the switches. But if you wanted to be 1000% certain, you could just set them so they are completely open or completely closed at all times and see if the problem persists. In either case, you would not see this behavior on a modern Stern. (Open at all times would be better because a closed switch might actually be opening and closing if the contacts are bad).

(But it sounds like this has been tested. You can see the switch is still open, even when the problem is happening. Please be *certain* that this is the case, because what you're experiencing is exactly what could happen if the EOS gap was too small).

If it were me I would just replace the transistors because that was the only thing that was changed. But that sucks if you have to send away the board to get it done.