There is a remote possibility that the thermocouples have been installed in the incorrect locations. This is more likely a potential cause if heaters or thermo-couples had been recently replaced on the machine. If this is the case you will see a zone that is running hot without calling for heat and another zone will be running cold while calling for heat continuously. This is a good indicator that the thermocouples have been cross wired. IR photos can be used to discover hot spots in the nozzle as shown in Figure 20.4.