I've had two bikes in over the last couple of weeks with rotation sensor failures. By rotation sensor I mean the sensor that tells the ECU the engine is running. It gets called all sorts of things - crank angle sensor, speed sensor, stuff like that. On the single pick up bikes, which began with the 1.6M ecu of 1995 and continue now with the 59M and 5AM, the sensor was originally in the LH engine or alternator cover, just at the front top edge of it, and triggered by the teeth on the timing gear. As per the red circle in the photo below. On the water cooled engines it's hiding behind the hose from the thermostat or lower radiator.
On the 1098 and later engines it has moved to the top of the crankcase just to the rear of the engine mounting bolt hole above the red circle above. It is still triggered by the timing gear.
Usually, when they fail, the bike stops. It might start again after it has cooled down a bit.
The first to arrive was an ST3. It would idle, but not rev without popping and banging. It arrived at the end of the day, so I took a little time to have a quick look see. It sounded very rich to me, so I checked the fuel pressure. It was a little high at 3.5 bar, but nothing outrageous. The diagnostic tool showed no errors, and all the sensor inputs were totally plausible. But with the engine running, the RPM was showing 0, which made me think rotation sensor. Given it was an ST3, and both side fairings had to come off due to the sensor being on the LH in the alternator cover, but the connector being zip tied to one of the RH frame rails, it got put to the side as it was late and I didn't have time to start pulling it apart.
The next one to arrive was an M400ie. Exactly the same symptoms, which surprised me, but also picked my interest immediately. Again, it arrived at the end of the day when I had finished what I had planned, so I had time to try the theory. Being an M400ie, the rotation sensor is in plain view meaning the swap test was simple. I measured the gear depth, measured my test sensor and popped it in with the original shim. Started instantly, and ran well enough to confirm the theory. It took a while for it to clear itself out, no doubt due to the residue of a heap of partially burnt fuel. But a road test showed it to be fixed.
I ordered a couple of sensors, and when they arrived pulled the sides off the ST3 and replaced its sensor. Thankfully, that was all it took, and it was ready to go again.
So, a new one on me, but an interesting one to see.