Has anything changed recently? How high is the temp during the alarm compared with the set point? What are the thermistors reading? When you say you have done temp calibration is this the temperature indication adjustment on service screen 7.4 or have you also done the amplifier zero and span adjustment on service screen 2?
The only thing I can think of that has changed would be the incoming water temperature.
It is hard to say what the actual temperature is compared with the set point as it only appear to happen at Home patients houses and it seems to start happenning intermittentely and then more frequently, as though it is a calibration drifting.
The thermistors all look to be reading the same temperatures.
I have carried out the zero and span adjustments, I have not tried the temperature indication.
When I spoke to Nikkiso about these errors I was told that the way they have been fixing them is by reseting the calibration data and doing all the calibrations, if this doesn't work they have been changing the CPU brd. Which seems to me to be a very expensive fix.
I was wondering about the incoming pressure regulator?
The only issue we had someone time ago with temperature was resolved by re-routing the tubing around the heater, it seemed that because they are close together they cause interference, but I cannot recall whether it was heat or electrical interference. Scott barlow is the man to ask.