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School Bus Emission Warning Lights

The HEST (high exhaust system temperature) warning light and the DPF (diesel particulate filter) or exhaust filter are the two main indicators that tell the operator when a manual regeneration is required. The ECU is programmed to initiate what is called a passive regeneration while the engine is running on the highway or in the city.

Some operators in our fleet mistaken the hot exhaust system temperature light as an engine code but all it does is warn them that the exhaust temperature is very hot and to keep the bus parked (when required) in a safe area. It is also a warning to keep people away from the exhaust coming out of the tail pipe. The HEST will flash on and off randomly as the operator does the route which is no cause for alarm.

The temperature during a parked regen can reach as high as 1200 degrees inside the DPF. The main objective is to cook out the soot build up in the Diesel Particulate Filter. The ECU is programmed to create as much heat in the exhaust system as possible. Mercedes uses a doser valve that shoots a fine mist of diesel down the exhaust pipe to create more heat in the DPF.

Cummins engines use DEF (diesel exhaust fluid) to cause a chemical reaction and reduce emissions in their patented SCR (selective catalytic reduction) system. Trying to recirculate crankcase gases into the intake along with the EGR (exhaust gases) is a huge challenge when it comes to burning clean exhaust out the pipe.

Emission Warning Light Sequence

When the DPF light comes on and stays on it indicates that it’s time to do a park regeneration. The operator needs to find a safe spot to park and keep people away from the tail pipe. This process takes at least 1/2 and hour maybe more depending how plugged up the exhaust filter is.

When the DPF light starts to flash the engine will derate indicating that the exhaust filter restriction is increasing and a parked regen must be done asap. In the third phase the DPF stays on along with MIL (malfunction indicator lamp) which is strictly an emission related fault light. There will be a further loss of engine power.

Finally when the phase four “red” shut down light comes on it’s tow truck time. The operator must “shut it down” before something blows up. The engine computer will not let the vehicle go any further to safeguard the engine and emission system. The lights on the dash are there for a reason to protect the engine and all the related emission accessories.