DPF Light On? Causes, Fixes and When to Stop Driving

Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent

An amber DPF light usually means your diesel particulate filter is filling with soot or a fault has developed. Both the RAC and the AA say about 10 minutes of driving at over 40mph can often finish the cleaning cycle and clear the light. If the light stays on, turns red or the car loses power, get it checked.

What the DPF does and why the light comes on

A diesel particulate filter (DPF) sits in the exhaust and traps soot so it does not go into the air. The AA says diesel cars built since 2009 must have one.

The soot has to be burned off at high temperature, which is called regeneration. This happens in two ways:

• Passive regeneration happens on its own during sustained faster driving, such as motorways and fast A-roads, when the exhaust gets hot enough.
• Active regeneration is triggered by the engine software, which adds extra fuel to raise the exhaust temperature. The RAC says this normally starts when the filter is about 45% full. The AA says it happens roughly every 300 miles and takes 5 to 10 minutes.

The DPF light comes on when the filter is getting blocked because regeneration has not been able to finish, or when the system detects a fault.

Common causes of a blocked DPF

The RAC names short, low-speed journeys as the main cause. If the car never gets hot enough for long enough, regeneration keeps getting cut short.

Other causes listed by the RAC and the AA include:

• The wrong engine oil. The RAC advises low-ash (low-SAPS) oil where specified, as some oil additives can block the filter. Always check your handbook.
• Missed services.
• Fuel system or EGR faults that make the engine produce extra soot.
• Other warning lights or stored fault codes, which can stop regeneration from running.
• Low fuel. The AA says active regeneration may not start if the tank is below about a quarter full.
• Low additive level. Some cars use a fluid additive to help burn off the soot. The AA says without it the DPF will block quickly.

Is it safe to drive with the DPF light on?

With an amber light and the car driving normally, you can usually keep going and try to clear it. The RAC says not to ignore an amber light for long periods.

Stop and get it checked if:

• The light turns red. The RAC says red lights need immediate attention and usually mean stopping as soon as possible.
• The car loses power or other warning lights come on.
• The light stays on after a proper run.

The AA warns that if you keep driving slowly in stop-start traffic, soot keeps building until the car goes into restricted performance mode, often called limp mode. At that stage, driving at speed is not enough. The filter then needs a forced regeneration using diagnostic equipment.

How to try to clear a DPF light

1. Make sure you have at least a quarter of a tank of fuel.
2. When it is safe and legal, drive for about 10 minutes at over 40mph. The RAC also suggests a 30 to 50 minute run at a steady speed on a motorway or A-road to help clear the filter.
3. Keep the drive steady and avoid stopping and starting the engine part way through.

While active regeneration is running, the AA and RAC say you may notice the cooling fans running, a higher idle, Stop/Start not working, higher fuel use, a changed engine note and a hot, acrid smell from the exhaust. This is normal.

If the light does not go out after a proper run, book a diagnostic check.

What affects the cost of fixing a DPF problem

It depends on why the light is on. Roughly in order, the options are:

• A good run that lets regeneration finish, which costs nothing but fuel.
• Fixing an underlying fault, such as a sensor or a problem making the engine sooty.
• A forced regeneration using diagnostic equipment.
• Removing the filter for specialist cleaning. The AA says ash cannot be removed with the filter in place.
• Replacement, which the AA describes as needed only in extreme cases. It says a DPF on a car used correctly should last well over 100,000 miles.

The make and model also affect the cost, which is why finding the cause first matters.

Can you just remove the DPF?

No. GOV.UK guidance says removing a DPF will almost always make a vehicle illegal for road use, with fines of up to £1,000 for a car or £2,500 for a light goods vehicle. A missing DPF, where one was fitted from new, is an MOT failure.

Since May 2018, the MOT also records a major fault on a diesel with a DPF if the tester sees smoke of any colour from the exhaust or finds evidence the DPF has been tampered with. The AA adds that removal could invalidate your insurance.

How we can help

We are a mobile mechanic based in Aylesham and we do warning light diagnostics and fault finding at your home or workplace. For a DPF light, we can come out, read the fault codes, find the cause and tell you what the car needs before anything else is done.

We cover Aylesham, Canterbury, Dover, Deal, Sandwich, Folkestone and the villages in between. Labour is £30-£50 per hour depending on the job. Parts are extra, and the price is agreed on WhatsApp first. Not sure what a light means? Our dashboard warning lights guide explains the common ones, and our engine management light post covers that light in detail.

Frequently asked questions

How long can I drive with the DPF light on?
Not for long. The RAC says do not ignore an amber light for long periods. If it turns red or power drops, stop and get it checked.

Does a DPF light fail an MOT?
A missing or tampered DPF is a fail, and so is visible smoke from a diesel with a DPF. Get any warning light checked before your test.

Need help? Send a WhatsApp to 07563 350335 with your car’s make and model and we’ll come to you.

Sources

• RAC: Diesel particulate filters, guide to DPFs
• AA: Diesel particulate filters, filter cleaning and problems
• GOV.UK: Diesel particulate filters guidance note
• GOV.UK: MOT rule changes, 20 May 2018