Handling The HEST Lamp Re-Gen (HEST Light On Dash)

There are more abbreviations. Than you can shake a stick at. When the new modern engines were developed so was a long list of abbreviations for the new components. HEST stands for High Exhaust System Temperature and the lamp it illuminates tells you that your vehicle is in regen mode.

The HEST lamp is illuminated when your vehicle is starting and going through a regen operation. This is a time when you have to keep the engine at low idle until the lamp goes out. You cannot be in high idle or PTO mode when your vehicle is doing its regen.

To learn more about the HEST lamp and how it operates just continue to read our article. It has the information you want to know about so you do not make the mistake of turning off your engine in the middle of the regen. Only in emergencies are you to turn your vehicle off in this mode.

HEST Light on The Dash


There have been times when drivers have been confused about the illuminated HEST lamp. They think that this lamp is telling them that a code has been set and there is a problem with the engine.

Their assumption would be incorrect. The HEST lamp is an information-only lamp that will tell you one of two things. When it is lit, it means that either a manual regen is required or the ECU has started a passive regen while the vehicle is in motion.

That is all there is to this lamp. There are no codes or repairs to worry about. When you see the lamp lit, you need to keep your exhaust pipe away from any combustible materials.

This happened to one driver just as he was pulling into the pumps to refuel. He had to drive around for about 20 minutes till the light went off so he could refuel and be on his way.

The reason you need to stay away from combustible materials, including any gas fumes, is that the ECU can generate up to 1200 degrees F inside the DPF to burn off the soot. The ECU is programmed to generate as much heat as possible

We have seen this information in manuals for emergency vehicles as well as for school buses. This information is not just for diesel pickup trucks or RVs.

HEST Indicator Light Peterbilt


The Paccar after treatment Systems operator’s manual simply restates what we said in the previous section. But not in so many words. Their silence on the HEST lamp tells you that this is just an information-only light that lets you know what is going on with your engine at the time.

The only instructions concerning this light are a series of warnings telling you to stay away from the exhaust pipe. Those warnings follow:

Temperature of the tailpipe, exhaust pipe, the diesel particular filter (DPF) / selective catalytic reduction (SCR) device and surrounding components including enclosures and steps, will be elevated during and shortly after a re-generation event or normal vehicle operation when engine is under high or heavy loading. If the High Exhaust System Temperature (HEST) warning lamp is on:

- Do not park in an area of combustible vapors or materials. You must keep combustibles at least 1.5 m (5 ft) away from the exhaust (outlet) stream (as it exits the tail pipe) while the HEST lamp is illuminated. Always park your vehicle outside. Failure to do so could ignite an explosion or harm bystanders which could result in serious injury.

- Do not park in an area where people are close by. You must keep combustibles at least 1.5 m (5 ft) away from the exhaust outlet while the HEST lamp is illuminated. Failure to do so could result in serious injury.

- Do not approach the exhaust system or surrounding areas without allowing adequate time for the system to cool down. Failure to do so could result in serious burns to the skin.”

All About Regens


There is not much more that can be said about the HEST light. But there is a ton of information about regens. There are different types of this normal procedure and they happen several times a day.

Passive Regeneration:

Passive regeneration uses heat from naturally occurring exhaust gasses to oxidize built-up particulate.

Active Regeneration:

An active regeneration cycle will be initiated if exhaust temperatures are not high enough to regenerate the filter. The motorhome must be traveling in excess of 20 mph for an active regeneration cycle to initiate. During an active regeneration cycle, diesel fuel is introduced into the exhaust system upstream of the DPF.

The fuel will ignite and super-heat the DPF to oxidize particulate matter. The HEST (High Exhaust System Temperature) dash light will illuminate when exhaust temperatures reach 1450º F., indicating that an active regeneration is underway and exhaust system temperatures are elevated above normal levels. These temperature levels may occur for up to 40 minutes. The HEST light will remain on until the exhaust temperature cools to 650º F., which may not be until the engine is turned off and the exhaust system cools before restarting. The HEST light does not indicate a fault as long as there are no other active warning lights.


If the engine is turned off during an active regeneration cycle, the cycle will automatically begin again (if necessary) when the motorhome is operated at speeds above 20 mph. (source)

You can read more about a parked regen at this link.

Some Additional Words

When you see this lamp illuminated, you do not have to do anything except follow those warnings. This is a normal operation that happens many times throughout any given day when you are driving.

All the regen does is burn off excess soot from the engine’s interior to keep it from polluting the air or harming your motor. Just do not turn your engine off when you see this lamp lit or it starts all over again when you start your engine.

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