I have a B&W pyrometer which had been working fine for several years. Over this last winter the truck sat for 3 months. Now when driving the pyrometer moves slow and only gets to 600 when I know under certain driving conditions it should reach 900. Got anther thermocouple and its reacts the same. So is there a test procedure, or expected voltage out of the thermocouple. I would expect the gauge to work or not, did not expect a 1/2 working failure mode

 

You need a thermometer or infrared temp gun that reads 800-1000°
Wrap the the thermocouple to a 100 watt light bulb with foil and compare the temps.