There are two versions of the elbow sensor on Onan generators on Super Maramus. 

Some Onan models have part 309-0295-01 which is Normally Closed. Other models have part
309-0295 is Normally Open. Which sensor you have depends on your Onan Model and Serial. Check the
parts manual for your model to determine how to test this sensor. Or call your Cummins/Onan dealer with
your model and serial number. Remember there are a number of sensors in the shutdown system, and
these switches will fail. I replaced the exhaust manifold temperature switch and the water-flow switch
because of an accumulation of debris inside. I also had one situation with a loose connection on a switch.
Did you use your laser temperature gun to measure temperature? The normal temperature of the exhaust
elbow is about 50C and the normal temperature of the coolant is about 80C. Were these in range? How
many hours since the last impeller change? Should be less than 300hrs or 12 months. The impeller blades
can look OK, but the impeller can be defective.