Trane Flame Sensor

I have had two calls on a Trane RTU, M #YSC120. The customer said it was not heating and had a fault, "Flame not sensed with a call for heat". Both times he reset it through Tracer. When I arrived, it was heating. I checked the manifold gas pressure, pressure switch tubing and flame sensor. The IOM did not have what flame signal it requires. I was reading between 5.6 to 6.6 microamps at the flame rod. I also checked another unit and it had just about the same flame signal. I cycled the burner at least 12 times and it fired each time. Does anyone know what it is actually looking for?

Thanks