The diesel particulate filter (DPF) is one of the most expensive components on a modern diesel car. When it fails, repair bills can run into thousands. But is a DPF problem grounds for rejection? Here's what you need to know.
What Is a DPF?
The Basics
The DPF is part of the exhaust system. It captures soot particles from the engine, preventing them from being released into the air. Periodically, the system "regenerates" – burning off the accumulated soot at high temperatures.
Why They Exist
DPFs became mandatory on diesel cars from 2009 (Euro 5 emissions standards). They're essential for reducing harmful particulate emissions.
How They Work
- Soot collects in the filter during normal driving
- When conditions are right (hot exhaust, sustained speed), regeneration occurs
- The soot burns off at temperatures around 600°C
- The filter clears and the cycle repeats
Common DPF Problems
Blocked/Clogged DPF
The most common issue. The filter becomes so full of soot that it can't regenerate properly.
Symptoms:
- DPF warning light on dashboard
- Loss of power (limp mode)
- Poor fuel economy
- Increased exhaust smoke
- Difficulty starting
Causes:
- Too many short journeys (regeneration can't complete)
- Faulty sensors or related components
- Engine problems causing excessive soot
- Previous owner driving habits
- Faulty EGR valve contributing to buildup
Failed Regeneration
The car attempts to regenerate but fails.
Symptoms:
- Warning lights
- Smell of burning
- Very hot exhaust
- Engine running rough
Physical Damage
The filter element can crack or break down.
Symptoms:
- Rattling noise from exhaust
- MOT failure
- Persistent warning lights
- Complete failure to regenerate
DPF Removed (Illegally)
Some owners remove DPFs to avoid problems. This is illegal for road use and an MOT failure.
DPF Repair Costs
Forced Regeneration (Garage)
A technician forces regeneration using diagnostic equipment.
- Cost: £100-200
- Effectiveness: Works if blockage is minor
Chemical Cleaning
Specialist cleaning using additives or off-car cleaning.
- Cost: £200-400
- Effectiveness: Can restore heavily blocked filters
DPF Replacement
New filter fitted.
- Cost: £1,000-3,000+ depending on vehicle
- When needed: Damaged filter or cleaning unsuccessful
Related Repairs
DPF problems often involve other components:
- Sensors: £100-300 each
- EGR valve: £300-600
- Injectors: £200-500 each
Total repair bills can easily exceed £2,000-4,000.
Is a DPF Problem Grounds for Rejection?
The Key Question
Was the DPF problem present at the time of sale, or did it develop afterwards due to your driving?
When You CAN Reject
The DPF was already failing at sale:
- Warning light appeared within days of purchase
- Problem existed before you drove it significantly
- Previous owner's driving patterns caused the blockage
- Fault codes show long-term issues
The dealer caused or knew about the problem:
- Cleared warning lights before sale
- Performed temporary fix that failed quickly
- Sold knowing the DPF was blocked
- Any DPF fault making the car unsatisfactory
- Doesn't matter if it's "your driving" yet
- Clear right to reject
When Rejection Is Harder
Problem developed from your driving:
- You've done months of short journeys
- Your driving pattern prevents regeneration
- The fault is genuinely new
After 6 months:
- You must prove the fault existed at sale
- More difficult with gradual DPF issues
The Evidence Challenge
DPF problems are tricky because:
- They can develop quickly from "wrong" driving
- Dealers claim it's always the driver's fault
- Proving pre-existing issues requires expertise
What Dealers Say (And Whether They're Right)
"DPFs fail because of how you drive"
Partly true. DPFs do need regular regeneration conditions (sustained highway driving). However:
- If the DPF was already marginal at sale, your driving just exposed the problem
- A healthy DPF should tolerate mixed driving patterns
- Excessive soot may indicate other engine problems
"That's not covered under warranty"
Often false. Many warranties exclude "wear and tear" but a failed DPF on a recent car isn't wear and tear – it's a component failure. Your statutory rights don't have these exclusions.
"You should have done more motorway driving"
Irrelevant for rejection. You're not obliged to drive a certain way. The car should be fit for normal use, including urban driving.
"The DPF was fine when we sold it"
Prove it. Ask for evidence. When did they last check it? What were the readings?
Getting Evidence
Diagnostic Check
A garage can read:
- DPF soot level
- Regeneration history
- Related fault codes
- How long issues have been present
Some fault codes are timestamped, showing when problems began.
Service History
Was the DPF addressed before sale? Any forced regenerations? Additive top-ups?
MOT History
Check for:
- Smoke test fails or advisories
- DPF-related comments
- Mileage patterns (lots of owners doing short journeys?)
Independent Inspection
An expert opinion on whether the DPF condition is consistent with:
- Pre-existing problem
- Recent deterioration
- Normal wear for age/mileage
Steps to Take
1. Document Immediately
- When did the warning light come on?
- How many miles have you driven since purchase?
- What driving have you done?
- Take photos of warning lights and mileage
2. Get a Diagnostic Check
Find out exactly what's wrong and, if possible, how long it's been developing.
3. Write to the Dealer
- State the problem
- Cite Consumer Rights Act 2015
- Request repair or rejection depending on timing
- Include diagnostic evidence
4. Don't Pay for Repairs Yet
If you pay for repairs, you may lose rejection rights. Let the dealer have opportunity to remedy first (unless within 30 days, when you can reject directly).
5. Consider Timing
- Within 30 days: Strong rejection right
- 1-6 months: Right to repair attempt, then rejection if it fails
- After 6 months: You need to prove pre-existing fault
Buying a Diesel: DPF Checks
Before Purchase
- Check for DPF warning lights
- Ask about driving patterns of previous owner
- Request diagnostic check of DPF soot level
- Look for evidence of regeneration problems (very clean or very dirty exhaust)
After Purchase
- Do a proper motorway run within first week
- Monitor DPF warning lights
- Report any issues immediately
- Keep evidence of your driving patterns
The Bigger Question: Should You Buy Diesel?
DPFs Are Getting Better
Newer systems are more reliable and better at regenerating in varied conditions.
But Risks Remain
For predominantly urban/short journey use, petrol or hybrid may be more suitable. DPFs and city driving don't mix well.
Value Impact
DPF problems affect resale value. A car with known DPF issues is worth significantly less.
The Bottom Line
DPF problems can absolutely be grounds for rejection if:
- The fault existed at the time of sale
- The car is unsatisfactory for normal use
- You're within your rights timeframe
The challenge is proving the problem was pre-existing, especially with a fault that's sensitive to driving patterns.
Act quickly, get diagnostic evidence, and don't accept the "it's your driving" excuse without scrutiny. A properly functioning diesel should handle varied driving – if it can't, something was already wrong.
Stuck with a diesel that's got DPF problems? Check if you qualify – we handle complex cases like DPF disputes regularly.
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