DPF Light On? Why Short Journeys Block Your Diesel Particulate Filter

What a DPF does

Most diesel cars built since around 2009 have a diesel particulate filter in the exhaust. It traps the soot that diesel engines produce so it does not come out of the tailpipe as black smoke.

The filter cannot hold soot forever. To empty it, the car burns the soot off at high temperature, a process called regeneration. When regeneration cannot happen, the filter fills up and the DPF warning light comes on.

Why short journeys are the problem

Regeneration needs the exhaust to get very hot and stay hot for a while, which normally means a steady run at motorway or dual carriageway speeds. A diesel that only does short hops around Oldbury, the school run or a few miles to work, rarely gets hot enough for long enough.

The car may start a regeneration and then have it interrupted when you switch off. Do that repeatedly and the soot level keeps climbing until the light comes on and the car may go into limp mode.

What you can try first

If the DPF light has only just come on and the car is driving normally, a 20 to 30 minute run at a steady speed, keeping the revs up in a lower gear than usual, will often let the car complete a regeneration on its own. Check your handbook, as some manufacturers give specific instructions.

If the light stays on after that, or the engine management light, glow plug light or limp mode appears as well, stop trying and get it looked at. Driving on a heavily blocked filter can damage the turbo and the filter itself.

Why DPFs block in the first place

Short journeys are the usual cause, but not the only one. A faulty pressure sensor, a sticking EGR valve, a leaking injector or the wrong grade of engine oil can all make a DPF fill faster than it should. Cleaning the filter without finding the cause just means it blocks again.

That is why we always start with a diagnostic check. We read the soot level and sensor data, look for the fault behind it and then advise whether a forced regeneration, a clean or a replacement filter is the right answer.

The MOT and removed filters

Removing a DPF is illegal on a road car and it is an MOT failure if the filter was fitted from the factory and is now missing or has been hollowed out. The examiner will also fail a diesel that puts out visible smoke on the emissions test.

Get your DPF checked in Oldbury

If your DPF light is on, call us on 0121 544 3736. We will diagnose why the filter is blocked, explain the options in plain English and give you a price before we start any work.