When a Diesel Particulate Filter regeneration process begins, the High Exhaust System Temperature HEST lamp illuminates and exhaust gas temperature may reach 1500 degrees F. That's hot enough to ignite and/or melt materials and burn people.
The main function of the HEST lamp is to alert the operator of extreme high exhaust system temperatures. It also common practice for the lamp to illuminate on and off during normal vehicle operation while the engine completes a regeneration process.The HEST lamp is only used for informational purposes, and does not indicate need for vehicle or engine service. If an emergency situation occurs always shut down the engine immediately.
*Courtesy of Mitchell1