On MX-Series router, the output of  ​'show chassis environment' command shows that "Exhaust" temperature is lower than the temperature of the intake.

Example:

user@router>show chassis environment
...
      CB 1 Intake             OK       28 degrees C / 82 degrees F
      CB 1 Exhaust A          OK       27 degrees C / 80 degrees F
      CB 1 Exhaust B          OK       33 degrees C / 91 degrees F

This does not look correct because the airflow comes from Intake, which takes some heat from CB components and goes out through Exhaust A/B. Therefore, the temperature of the airflow on the exit should not be lower than the temperature on the entrance.

This is not an indication of any problems. The temperature sensors tend to be influenced strongly by what other components are near them on the board and how hot those components are running. On the SCB that is used on MX240/480/960 the inlet temperature sensor is placed right next to a big ASIC component that runs fairly warm. Hence, the inlet temperature reads a little high. On the other hand, the Exhaust A temperature sensor is located in an area of the CB where there are very few active components. Even though it is labeled ‘exhaust’ because it is on the top edge of the board which is closest to the air exhaust, it actually runs cool.