DPF Problems: Warning Lights, Causes and Solutions

What the DPF does and how regeneration works

The diesel particulate filter traps soot from the exhaust so that it does not escape into the air. Over time the filter fills up, and the car needs to burn that soot off. This is called regeneration.

During a regeneration the engine management raises exhaust temperature, often by injecting extra fuel, and the soot is burnt away to ash. This needs the engine to be hot and the car to be driven at a steady speed for a while. You might notice the cooling fan running, a slightly higher idle or a change in exhaust smell.

Warning signs of a blocked DPF

When regeneration cannot complete, soot builds up and the car warns you. The dashboard light is the main sign, but others include reduced power or limp mode, higher fuel consumption, a fan running more than usual and, in some cars, engine oil level rising because fuel is leaking past the pistons during failed regenerations.

Do not ignore a DPF light. Often a longer steady drive helps, but if the light stays on, or the car goes into limp mode, have it looked at before the problem gets worse. Check your handbook for what the light means on your car.

If a DPF light appears, a sensible first step is a steady drive of some distance once the engine is warm, in a safe place where you can hold a consistent speed. Avoid repeatedly switching the engine off during a regeneration, which you may notice by the fan running or the engine sounding different after you stop.

• DPF or engine warning light on the dash
• Loss of power or limp mode
• Fan running or idle revving after you stop
• Rising fuel use
• Oil level creeping up between services

What causes DPF problems

The most common cause is a driving pattern of short trips, which never lets the filter get hot enough to regenerate. Other causes are faulty sensors, such as pressure or temperature sensors, a failing EGR valve, injector problems, a turbo fault, or the wrong engine oil, which can leave excess ash.

Because the DPF is part of a system, fitting a new filter without finding the cause can lead to a repeat blockage. Diagnosis is key.

Engine oil also matters. Diesels with filters need oil of the specification in the handbook, because the wrong type leaves more ash that cannot be burnt off. Stick to the service schedule too, as a neglected engine produces more soot to begin with.

DPF diagnostics, cleaning and repairs

DPF diagnostics starts with reading the fault codes and live data, such as the pressure across the filter and the soot load. The results show whether the filter is simply full, whether a sensor has failed, or whether something else is stopping regeneration.

Depending on the result, the answer might be a forced regeneration, a DPF cleaning process, a sensor repair or a fix elsewhere in the engine. A filter that is badly blocked or physically damaged may still need replacing. At Banks of the Tyne in Hebburn we offer DPF diagnostics and DPF cleaning and repairs, and we will explain what is realistic after the inspection.

Why replacement is not always needed

Soot is the main thing clogging a filter, and soot can often be removed. If the filter is intact and the cause of the blockage is fixed, cleaning can be a sensible route. Replacement is for filters that are cracked, melted or contaminated beyond cleaning, and a garage should be able to explain which it is.

Do not remove a DPF. Taking it off a car that was built with one is an MOT failure, and it may break the law on emissions. Check the current rules if you are unsure.

Frequently asked questions