What Does the Diesel Particulate Filter Warning Light Mean?

In order to combat exhaust emissions, modern diesel engines are fitted with a particulate filter in the exhaust. This filter is designed to catch any larger particles of soot that can come from the diesel exhaust. Like all filters, this diesel particulate filter will also clog up, eventually restricting the airflow and reducing the performance of your vehicle. As a result, auto manufacturers have designed a few ways to “regenerate” the filter by raising the temperature in the exhaust to burn the soot and reduce it to ash.

When driving at high speeds, the temperature in the exhaust gets hot enough to burn the soot. This is called passive regeneration. If the vehicle doesn’t reach high speeds frequently, the engine can burn some extra fuel to raise the temperature at lower speeds and achieve the same result. This is called active regeneration. Vehicles use a combination of both regeneration methods to maintain the filter for as long as possible.

What the diesel particulate filter light means

As you drive, the car’s computer constantly monitors the soot level inside the filter to determine when regeneration is necessary. When the soot level reaches a certain threshold, usually around 45%, the engine will actively try to reduce the levels. If the engine isn’t able to burn the soot due to a short trip, the computer will then turn on the filter warning light to warn the driver that the soot level is building up. When you first see the light, you can resume normal driving, but when you get a chance, take the car onto the highway and drive it for around 10 minutes at 40+ MPH. This should sufficiently raise the temperature to levels high enough to regenerate the filter, and hopefully the light will turn off.

If you ignore the light, the soot will continue to build up until the computer turns on a “limp” mode to try and prevent any damage from occurring. This limp mode limits your top speed to reduce how much soot is getting sent through the filter. This is a last resort solution that the engine uses to try and save the filter. If your vehicle is in limp mode you may be able to save the filter, but you will need to use a scan tool. Take the car to a certified technician who can run a forced regeneration program to try and save the filter. If you wait too long in limp mode, the soot will build up too much and the forced regeneration will no longer be an option. Be sure to act quickly if your car ever goes into limp mode otherwise the filter will need to be replaced and that turns out to be quite a costly repair.

There are a few things that can prevent the regeneration process from happening while you are driving. Using the wrong engine oil or not having enough fuel in the gas tank can prevent active regeneration from occurring. Any engine codes present can also prevent the regeneration from occurring and in this case, the check engine light is likely to be illuminated as well. If you are trying to turn this warning light off by driving, make sure none of these factors could prevent the regeneration process from taking place.

Is it safe to drive with the diesel particulate filter light on?

When this light first comes on, continue driving your car like normal and try the active regeneration method by driving on the highway. This should turn off the light as long as nothing is preventing the regeneration from taking place. If the light doesn’t turn off, take the car to a certified technician to have the issue investigated further.

As mentioned earlier, waiting too long with this light on will force the engine into limp mode at which point you should not drive the car until the issue has been remedied. Not only is it unsafe for driving at reduced speeds, the soot will continue to build up until the filter needs to be replaced entirely. Why wait when you can save yourself time and money by acting quickly if the light ever comes on! Our certified technicians here at YourMechanic are always available to inspect your car and find out the cause of the issue.


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