Answer:
Both technicians are correct.
Step-by-step explanation:
Exhaust gas recirculation (EGR) contributes to limit the production of polluted gas mixture during internal engine combustion via an EGR valve. With the help of Exhaust gas recirculation (EGR), nitrogen oxides production is reduced drastically especially when the engine runs on diesel. Black smoke, engines making knocking sounds could be a clear indication of a bad EGR valve especially when the valve passage is blocked due to carbon deposits, or dirt from fuel blocking the valve opening thereby decreasing the power output, or in the case of the knock sounds, the fuel gets ignited early when the machine's operation at a given time is low. So, the Technicians are correct with the statements they made.