How to know if DPF is blocked

How to know if DPF is blocked

If you drive a diesel vehicle, it is likely fitted with a DPF, especially if it was made after 2009.

In this post, we’ll cover how DPFs work, the symptoms of a blocked filter, and what to do when your diesel particulate filter is clogged to avoid serious car problems. Ready? Let’s get started! 

What is a DPF?

A DPF filter prevents harmful particulates from exiting the engine via the exhaust system. The primary particulate it stops is soot, a by-product of diesel fuel.   

The DPF traps soot and holds it until it can be transformed into ash via regeneration - a process which requires the DPF to be extremely hot.   

The main benefit of burning soot into ash is that the latter is much less harmful to humans. Soot stays in the air we breathe for a long time, making up most of inner city air pollution.  

Fun Fact: Since 2009, DPFs have been mandatory on diesel cars. 

How does a DPF work?

DPFs work through filters that trap the soot and keep it until it burns away through regeneration. Again, this is an extremely hot process. The filter has to be at nearly 500°C. Ouch.  

The overall aim of this is to reduce soot emissions and pollutants in the air we breathe. 

The exhaust only reaches these temperatures at high speeds and engine loads. So, if you don’t drive at high speeds often, you won’t burn off any ash. At least 30 to 45 minutes of motorway or A-road travel is needed to burn the soot.  

The regeneration process can be split into two parts: active and passive regeneration. Here’s a quick summary of each:  

1. Passive Regeneration 

Passive regeneration happens during normal usage (motorway or A-road journeys) and the car’s computer doesn't have to intervene.  

2. Active or Forced Regeneration  

When the car isn’t driven at high speeds, the car’s ECU (Electronic Control Unit) begins an active regeneration cycle.  

Active regeneration involves the engine altering the fuel/air mixture to make the engine produce more heat from the exhaust. This is not very fuel efficient but can help to remove built-up soot.

Some cars even use a special catalyst solution stored in a tank.

Why is my DPF warning light illuminated?

An orange DPF warning light usually displays if the diesel particulate filter is clogged. This has an easy enough fix. Simply take the car on a journey at a constant speed so the DPF can turn excess soot to ash.  

If the warning light is red, it could be a more serious problem or DPF damage. Consult a professional.

What is a Blocked Diesel Particulate Filter?

A common fault with any DPF system is the filter blockage due to excessive soot build-up.   

One of the most cited reasons against choosing a diesel-engined car is that it needs to be driven long distances. This is partly due to the DPF; another common cause is the EGR valve.

As mentioned, the DPF must reach temperatures of approximately 500°C for the soot to be burnt off. If the vehicle is only driven for short periods, the passive regeneration of the DPF will not occur.   

The ECU will inevitably try to force a regeneration, but this can only be completed if the car is at a specific temperature and will be driven for a long time after the regeneration. This is often not the case within cities and with infrequent use.

8 Blocked DPF Symptoms

You can tell your filter is blocked when the orange DPF light flashes on your dashboard. However, this might not be the only sign your filter needs to be unblocked.

8 more common DPF symptoms include:

  1. DPF warning light on the dashboard  
  2. Engine low on power  
  3. The engine cannot rev to high RPMs
  4. Issues starting
  5. Black smoke from exhaust upon acceleration
  6. Strong odour of diesel fuel in the cabin  
  7. Reduced fuel economy
  8. Start-stop function not working

How to avoid a clogged DPF

Luckily, you can avoid a clogged DPF. Here are the top 3 tips drivers need to know to ensure their diesel particulate filter isn’t clogged.

1. Don’t only drive short distances

Sadly, diesel cars aren’t great if you only want to pop to and from the shops.  

The exhaust needs to be hot to burn off the soot. If it never reaches a temperature high enough, it will only become more blocked.  

To mitigate this, ensure at least once a week it makes a journey above 40mph for over 30 minutes, or more if possible. This is your sign to hit the motorway!

2. Use the correct oil

Special oils have been developed for use in DPF-fitted cars. These have lower element content to avoid soot buildup (e.g. Sulphated Ash, Phosphorus and Sulphur). 

You'll find these labelled low SAPS at any regular shop selling engine oil. For more information about oils, check out our guide here

3. Take care of any other engine issues promptly

Diesels are generally more complex than petrol engines. They have more components that can fail and cause issues with DPFs.  

You should pay close attention to any issues on:

The above can cause excess oil or soot to exit the engine through miss-fueling or oil consumption. These will clog up a DPF in record time, so be on the lookout for dash warning lights and act quickly.

All About DPF Regeneration

Have a few questions about DPF regeneration? Here are the most common queries about the process. 

How to tell your car is regenerating its DPF?

  • High Idle speed - the engine will remain at 1500-2000 RPM whilst stationary  
  • Excess heat from under the car 
  • Very poor fuel economy - instant economy readout will likely halve  
  • Warning message on the dashboard

If any one or more of these symptoms are present, you should consider how you continue your journey.  

What to do if your DPF is being regenerated?

It's wise to read your owner's manual regarding this before it is necessary, but the general advice is as follows:  

  • Continue to drive on roads that permit at least 40 mph
  • Drive for at least 30 minutes or until the DPF light has gone away
  • Do not turn off the car or allow it to idle for a long time
  • Be cautious of your fuel level. Regens use more fuel than regular running

Some manufacturers are different, so it's best to check the specific signs and dashboard lights your car will display, especially if you feel your DPF is clogged.

What to do if your DPF Regen fails?

If you cannot continue driving or get stuck in traffic, the ECU may cancel the regen. This will mean that most of the soot build-ups will remain, and the DPF needs to be regenerated soon.  

Try to schedule a drive at high enough speeds to clear the DPF. Continual driving at slow speeds will worsen the problem and may require manual DPF cleaning, that is, removing and chemically dissolving the soot buildup.

What if your DPF cannot be regenerated?

If the regen process has failed multiple times, especially under ideal conditions, your DPF may need to be cleaned by hand or replaced altogether. Usually, this involves removing the DPF and soaking the built-up soot with a solvent.

Diesel Particulate Filter Replacement Cost

Some garages specialise in replacement DPFs, but generally, most normal garages can remove and replace a DPF. They can cost anywhere between £800 to £2,000.

Spread the cost of your repairs into monthly payments with Bumper Car Repair Loans. Find 1000s of dependable garages and dealerships across the UK and apply for a credit limit of up to £5,000.


Common DPF questions

Still unsure how DPFs work? Want to know how your DPF affects your power? Check out common FAQs below.  

Can a blocked DPF cause power loss?

Yes. A blocked DPF can cause a loss of engine power. This is because the engine needs to get exhaust gases out as quickly as possible to allow air and fuel in. Blocking the exhaust can restrict the amount of fuel and burned air.   

Modern diesel engines have a turbocharger, which takes wasted energy from the flow of exhaust gases and uses it to force higher volumes of air into the engine, improving efficiency.

If the DPF is blocking the airflow through the exhaust, the turbo will not be spinning fast enough to create sufficient pressure, further reducing the diesel engine's power. 

Can a DPF cause low engine RPM?

Yes, alongside the engine not producing much power, it may not be able to rev very high. This is a consequence of both the exhaust gases restricting the engine's usual cycle and the lack of turbo pressure.  

As engines make the most power at a specific RPM range, all three of these issues compound, making the car feel extremely sluggish on acceleration.  

Doesn’t DPF regeneration solve blocking issues?

The car’s ECU will detect when a DPF regen is required and start one when the conditions are met. Driving the car incorrectly whilst a regeneration is in progress can cause the DPF even more problems. 

Does my car have a DPF?

If your diesel car was manufactured after 2009, it will have a diesel particulate filter.  

It wasn’t until the new Euro 5 regulations in 2009 that all diesel vehicles sold in Europe required a filter to reduce overall emissions. Many manufacturers added them in advance. The PSA group (Citroen and Peugeot) were the first to include DPFs as standard.  

Diesel Particulate Filters are a relatively new addition to the average diesel vehicle in the UK. They have been used in off-road diesel vehicles since 1980, such as forklifts and excavators. HGVs and trains have also used them for many years.  

The best way to check is to look in your owner’s handbook for reference to DPF regeneration or similar.  

Related Reading: What Is AdBlue Used For In Diesel Cars?

Do petrol cars have a DPF?

No, petrol cars don’t have a DPF. But they do have an OPF (Otto Particulate Filter). This has been mandatory since 2018 and works to reduce harmful matter from the exhaust pipe. Newer cars have quieter OPFs than older models. 

How long does a DPF last?

If maintained well and driven for longer journeys, a DPF should last up to 100,000 miles. So, don’t put off those road trips.  

Is it illegal to remove a DPF?

Yes, it’s illegal, and you’ll face an invalidated insurance policy and a fine (up to £1,000 for cars and around £2,500 for vans). Even if your diesel particulate filter is clogged, you should never remove it.

Blocked DPF Symptoms & Fixes: The Takeaway

Understanding how the DPF works is critical for all diesel owners. While you hopefully won't face this pesky issue, knowing how to navigate it will help you save time if it occurs.

If you need to fix your DPF and are worried about costs, split your bill into monthly payments at no extra cost and find a trusted partner with Bumper.

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