Red DPF light, or the car's down on power? Don't push it
If the car has lost power, other warning lights have joined in or you can smell something hot, get off the fast road and stop somewhere safe. A car in reduced power mode can't keep up with traffic. If the light's red, leave the clean-out drive alone: Land Rover, for one, says red means the filter is full and to get in touch as soon as possible. During a burn-off the filter can run at around 600°C, so don't stop on long dry grass. If anyone is hurt or in danger, call 000.
Why has the DPF light come on?
The filter catches soot so it doesn't go out the tailpipe. Every so often it has to burn that soot off, which is called regeneration. RACQ explains that this can happen by itself at highway speed, or the engine computer can add extra fuel to heat the filter to around 600°C.
That burn needs time. Once it starts, RACQ says it must be allowed to finish, which typically means five to 25 minutes above a set speed, usually about 80 km/h. Stop-start traffic and low speeds can stop it starting or finishing.
So the usual cause is how the car's being used. RACV says the filter can block when a diesel mostly does short trips. A diesel that only does the school run and the shops never gets hot for long enough.
How fast, and for how long?
| Speed | Time | |
|---|---|---|
| RACQ's typical figures | Usually about 80 km/h | Five to 25 minutes |
| Land Rover's Australian advice | 60 to 112 km/h | 20 minutes |
| CarsGuide's example | Over 50 km/h | At least 15 minutes |
The figures differ because the cars do, so your owner's manual wins. Use the speed it gives, and keep to the speed limit.
How to try clearing it
Check the colour, then the manual
Amber is usually the request for a drive: Land Rover says amber means regeneration is needed. A red light or a service message can mean something else on your car, so read that page of the manual first.
Pick the right road
You want a freeway or open highway where you can hold a steady speed legally. Go when you won't get stuck in peak-hour traffic.
Hold it steady and don't switch off
Stopping the engine halfway can leave the job unfinished. Fuel use can rise a little while it runs, which Land Rover says is temporary.
Watch what the dash does
On some cars a green light shows the job's done. On others the warning simply goes out. Some diesels have a dash switch that starts the clean: use it only the way the manual says.
Try it once, then stop
If the light's still on after a proper run, more laps won't help. RACV says warnings that persist need a mechanic.
Why are the DPF light and the engine light both on?
That combination changes things. RACV says a check engine light alongside the DPF light can mean another fault is stopping the filter from regenerating, and that's the fault to fix first. More highway driving won't get around it.
It's also how limp mode arrives. RACQ says that without regeneration the car drops into a reduced power mode with both lights on. See limp mode and engine light on.
What will the mechanic do?
First they read the fault codes and the soot level with a diagnostic tool, because the full filter is usually the symptom. The question is why it couldn't clean itself: a sensor, a fault elsewhere in the engine, or too many short trips.
If the filter is healthy, they can start a forced regeneration from the tool with the car parked. RACQ notes this may also mean changing the engine oil and filter, because the extra fuel can get into the oil.
Filters don't last forever. Ash from burnt engine oil can't be burned off the way soot can, so over time a filter may need cleaning or replacing. That's a much bigger job than a regeneration, so ask to see the reading that shows it's needed.
A mechanic who works on diesels every week suits this, because a forced regeneration needs software that talks to your make. Our page on exhaust and DPF work has more on what's involved.
What to ask the mechanic
- What stopped it regenerating?
- What's the soot level now, and what was it before you started?
- Does the oil need changing after the regeneration?
- Can the filter be cleaned, or does it really need replacing?
- What should change about how the car gets driven?
What you can try, and what needs a mechanic
You can
- Read the DPF page in the owner's manual.
- Do the drive it describes on a suitable road, within the speed limit.
- Note how often the light comes on and what trips you've been doing.
- Give a short-trip diesel a regular run at highway speed.
Leave to the mechanic
- Reading fault codes and soot levels.
- Finding why the filter couldn't clean itself.
- Forced regeneration, and any oil change that follows.
- Cleaning or replacing a filter that's reached its limit.
Our take
Amber light, car driving normally: we'd read the manual and do the drive that day, not next week. If it doesn't clear, or it's red, or the engine light has joined it, we'd stop trying and send the job. And if this keeps happening, we'd look at the trips before the filter. RACQ is blunt that a diesel spending its life in slow traffic will keep having this problem, so a regular highway run may be the real fix.