Why Short Trips Are Killing Your Diesel (and What That DPF Light Means)

Okay, "killing" is dramatic. But this is genuinely one of the most common things we see on modern diesels, and it's almost always caused by nothing more than how the car gets driven.

If your diesel does the school run, the shops, and a five minute trip to work and back, this one's for you.

What the DPF actually is

Every modern diesel has a diesel particulate filter in the exhaust. DPF for short.

Its job is to catch the soot before it goes out the back of the car. That's it. It's a filter, and like any filter it fills up.

The clever part is that it's designed to clean itself out. It just needs the right conditions to do it.

How it cleans itself, and why heat is the whole story

The filter gets rid of the soot by burning it off. That's called a regeneration, or a regen if you've heard someone at a workshop say it.

To burn soot off you need serious heat in the exhaust, and you only get that when the engine has been working properly for a decent stretch. On a good run at a steady speed it happens quietly in the background and you never know about it.

If the car doesn't get that run, it will try to force one. The engine management puts extra fuel in to raise the exhaust temperature and burn the soot off deliberately. That takes a good fifteen or twenty minutes of steady driving to finish.

Here's where short trips come in. You pull into the driveway eight minutes in and switch the car off. The regen doesn't finish. It starts again next time and gets cut short again. Soot keeps building up, and the filter never gets back to clean.

Do that for long enough and the filter blocks.

The light is not a "stop the car" light

When the DPF light comes on, the car is telling you it hasn't been able to finish a clean and it needs a decent drive.

So give it one. Get it out on a main road, hold a steady speed, and let it run for twenty minutes or so without stopping and starting. A lot of the time that's the whole fix, and the light goes out on its own.

What you don't want to do is ignore it for months. If the filter keeps loading up, the car eventually puts itself into limp mode to protect itself, and at that point you're not driving it out of the problem.

The one that catches people out

When a regen keeps getting interrupted, some of that extra fuel washes past the rings and ends up in the engine oil.

The oil level on the dipstick goes up rather than down. It thins the oil out, and thin oil doesn't protect the engine the way it should. If you've ever looked at your dipstick and thought that's odd, there's more oil in there than there was, that's worth a phone call.

That one isn't about the filter any more. That one's about the engine.

They don't last forever, even when you do everything right

Soot burns off. Ash doesn't.

Every diesel makes a small amount of ash that stays in the filter permanently, so even a car that lives on the highway and regens perfectly will eventually fill up. That's normal and it's a long way down the track. It just means a DPF is a part with a life on it, not one that lasts as long as the car.

Looking after it is mostly boring stuff. The right oil to the right specification, because the wrong grade makes ash faster. Decent fuel. Keeping up with your services. And if you've got a catch can fitted, keep it drained, because a blocked one causes its own problems.

On deleting it

We get asked. The answer's no.

Removing or disabling the DPF on a road registered vehicle isn't legal in Australia. It's an emissions control device fitted by the manufacturer, and taking it off makes the car non-compliant. There are fines attached, and it can affect your registration.

We're an authorised inspection station, so compliance isn't something we're loose about. We'd rather sort out why yours is blocking.

What to do if yours is playing up

If the light's been on a while, or it's come on more than once, bring it in. We'll scan it and tell you where it's actually at, because the car records a lot more about its regens than the dashboard shows you.

If it needs one, we can run a forced regeneration here. That's where we use the scan tool to make the car do a full burn off properly, under control, rather than hoping it gets there on a drive. From there we'll be straight with you about what it needs, whether that's the forced regen, a service, or something bigger.

And if your diesel genuinely only ever does short trips, that's worth knowing about too. It doesn't mean you bought the wrong car. It just means the odd longer run is doing more for it than you'd think.

Give us a ring on (08) 9752 4466, or book it in online at busseltonautorepairs.com.au/contact. We're open Monday to Thursday, 7am to 5:30pm.