On apparatus equipped with a diesel particulate filter (DPF), which of the following lights up when the exhaust system is very hot, usually due to an active regeneration in process?

The light that illuminates when the exhaust system becomes extremely hot, typically due to an active regeneration process in a vehicle equipped with a diesel particulate filter (DPF), is indeed the High Exhaust System Temperature indicator. This indicator serves a crucial safety function by alerting the operator that the temperature of the exhaust system is elevated, which can occur during the regeneration process where excess soot collected in the DPF is burned off to maintain the effectiveness of the filter.

During active regeneration, the exhaust temperature can rise significantly to facilitate the burning off process of accumulated particulates. The High Exhaust System Temperature indicator warns the operator to be cautious and to monitor the situation to prevent potential damage to components or a fire hazard. Understanding the function of this indicator is important for safe operation and maintenance of the apparatus.

The other options do not serve the same purpose. The DPF indicator would typically indicate issues related to the DPF itself rather than the current temperature of the exhaust system. The manual regeneration switch is a feature that allows operators to manually trigger the regeneration process, but does not indicate temperature. Similarly, the regeneration inhibit switch is used to prevent regeneration under certain conditions, but does not provide real-time feedback about exhaust system temperatures.