The plug of thermocouple is confirmed connected correctly at the time. I have no a good thermometer to calibrate my thermocouple but it does indicate the readings close to ambient temperature before engine starts. I think the reason could be length of thermocouple end inside the nozzle reduced due to vibration. The other reason may be that the temperature of cool spot is detected in the exhaust nozzle. Maybe two more thermocouples installed can get average EGT but ECU does not calculate.