DPF Faults, Cleaning
& Additive Services
Engine light on? DPF warning showing? Losing power? We diagnose the real cause first — then fix it properly.
What is a DPF and What Does It Do?
If you drive a diesel car made after 2009, it has a Diesel Particulate Filter — but most people have no idea what it actually does until the warning light comes on. Here's a plain-English explanation.
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Diesel engines produce soot
Every time your diesel engine burns fuel, it produces tiny soot particles — so small they're invisible to the naked eye, but harmful to breathe. Without a filter, they go straight out of the exhaust into the air.
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The DPF catches the soot
The DPF sits in your exhaust system and acts like a honeycomb-shaped sieve. Exhaust gases pass through, but soot particles get trapped in thousands of tiny channels inside the filter. Think of it like a very fine air filter for your exhaust.
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It cleans itself — when conditions allow
Once enough soot has built up, the DPF burns it off at very high temperatures — a process called regeneration. This happens automatically on a long motorway run, where the exhaust gets hot enough. On a short school run or town commute, it never gets hot enough to do this.
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When it can't clean itself, it blocks
If the DPF fills up with soot it can't burn off — because of short trips, a fault elsewhere in the engine, or the wrong oil — it blocks. This triggers warning lights, causes a loss of power, and can put the engine into limp mode. Left too long, it can mean an expensive replacement.
How Regeneration Works
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Passive Regen Happens automatically on faster roads. Exhaust temperature rises enough to burn soot without the car doing anything extra.
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Active Regen The ECU detects the DPF filling up and injects a little extra fuel post-combustion to raise exhaust temp. You won't notice this happening.
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Forced Regen If the DPF gets too full for either of the above, a garage uses diagnostic equipment to command a regen cycle at safe conditions. This is what we do.
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Physical Clean When a forced regen won't work — because ash (not just soot) has built up, or the blockage is too severe — a chemical or back-flush clean is required.
Seeing Any of These?
These are the real warning light symbols that appear on UK diesel dashboards. Here's what each one actually means.
DPF Warning
Filter full of soot — drive at speed or book a diagnostic regen.
Check Engine
Amber engine light — almost always present with a DPF fault. Needs a scan.
Glow Plug
Faulty glow plugs cause incomplete combustion — a leading cause of DPF blocking.
EPC / Limp Mode
Electronic throttle fault — vehicle is likely in limp mode.
Red Engine Light
Stop driving immediately. Serious fault — risk of major engine damage.
AdBlue critically low — vehicle may refuse to start if ignored.
⚠ A red warning light means stop and call us — do not continue driving. 01438 232556
A Blocked DPF is Usually a Symptom,
Not the Root Cause
This is the most important thing to understand. Simply cleaning a blocked DPF without diagnosing why it blocked will almost certainly result in it blocking again within weeks. We always investigate the root cause first.
Failed Glow Plugs
Glow plugs are essential for complete combustion, especially in cold weather. When they fail, fuel doesn't burn properly, pushing excessive raw soot into the DPF far faster than normal — preventing regeneration from ever completing. A very common hidden cause.
Split or Loose Boost Pipe
A boost pipe leak drops turbo pressure, causing the engine to run rich and produce excess soot. One of the most common causes of repeated DPF failures — and it will trigger fault codes alongside the DPF light. Often a surprisingly cheap fix that saves a costly clean.
EGR Valve Fault
A faulty EGR valve stuck open or closed disrupts the combustion balance, flooding the DPF with soot. The DPF sensor will flag this, but the EGR is the real cause. Cleaning the DPF without sorting the EGR is a short-term fix at best.
Injector Problems
Worn or leaking injectors deliver fuel inefficiently, creating excess unburnt carbon that clogs the DPF. An injector returning too much fuel can also cause oil dilution — a serious secondary problem that goes well beyond the DPF itself.
Short Stop-Start Journeys
Regeneration needs sustained driving at temperature — typically 20+ minutes at motorway speed. Short school runs or town commutes never get the exhaust hot enough. Over time, soot accumulates until a full blockage occurs. Very common and entirely preventable.
Faulty DPF Pressure Sensor
The differential pressure sensor tells the ECU how full the DPF is. If it fails, the car may attempt regeneration at the wrong time — or never at all. A perfectly good DPF can fill with soot unnecessarily because the car simply doesn't know it needs to clean itself.
Exhaust Gas Temperature Sensor Fault
DPF temperature sensors monitor the filter during regeneration. A faulty sensor gives the ECU false readings, causing it to abort regen cycles early — or not start one at all. The DPF fills slowly and silently until a warning is triggered.
Wrong Engine Oil Used
Diesel vehicles with DPFs must use Low-SAPS oil (typically 5W30 C3 spec). Standard oil increases ash deposits inside the DPF which — unlike soot — cannot be burned off in a regen. This permanently reduces DPF capacity and requires a physical clean to resolve.
Three Levels of DPF Cleaning
We offer three cleaning methods to suit different levels of blockage and budget. Not sure which you need? Our diagnostic check will tell us exactly what's required.
Aerosol Treatment
A professional-grade aerosol cleaner administered directly into the DPF. Best for mildly blocked filters or as a preventative treatment when a warning has just appeared and the blockage is early-stage.
- Fast, same-visit treatment
- DPF stays fitted to the vehicle
- Best for early-stage or mild soot build-up
- Most affordable option available
Chemical Clean
Our most recommended option. A specialist chemical cleaning solution dissolves soot and carbon deposits thoroughly. Excellent results for most blocked DPFs at a fraction of the cost of a full removal and refit.
- Best balance of price and results
- Professional-grade cleaning solution
- Effective for moderate to heavy blockages
- DPF pressure sensor check included
- Fault codes cleared on completion
Remove & Back-Flush
For severely blocked DPFs, we remove the filter completely and use specialist equipment to back-flush all soot and ash from the substrate — restoring it as close to new as possible.
- Complete removal of soot and ash
- Back-flushed using specialist DPF machine
- Best for severely blocked filters
- Closest result to a new DPF
- Flow test confirms successful clean
Full Diagnostic Check From £45
Cleaning a blocked DPF without knowing why it blocked is like mopping a floor with a tap still dripping. We always diagnose first.
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Full multi-system fault code scan
We read all codes across every module — not just engine codes. This reveals hidden faults causing the DPF to block.
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Live data & DPF soot level check
We read the DPF differential pressure, soot load, ash level, regen history, and temperature sensor data in real time.
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Physical inspection
We check for boost leaks, EGR fouling, oil consumption, and any other mechanical contributors before recommending a clean.
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Written report & fixed quote
Plain-English explanation of exactly what's wrong, and a fixed price before any work begins. No surprises.
- Full multi-system fault code scan
- DPF soot load & ash level read
- Live sensor data analysis
- Regen history check
- Boost & EGR inspection
- Written report with fixed repair quote
Diagnostic fee may be deducted from repair cost
Common DPF Fault Codes Explained
Had your car scanned and been given one of these codes? Here's what they mean in plain English — and why they need proper investigation, not just a code clear.
DPF Efficiency Below Threshold (Bank 1)
The ECU has detected the DPF is no longer filtering soot effectively. Most commonly caused by a heavily blocked or damaged filter. Often the first code people find when searching their DPF fault.
DPF Particulate Mass Too High (Bank 1)
Soot load inside the DPF has exceeded the maximum threshold. Regeneration is failing and the filter needs professional attention. Often appears alongside P2002.
DPF Regeneration Not Active / Ash Restriction
Regeneration has stopped or never completes. Can indicate excessive ash build-up (not just soot), a faulty sensor, injector issue, or exhaust leak. Ash cannot be burned off — physical cleaning is required.
DPF Differential Pressure Too Low (Bank 1)
Pressure across the DPF is lower than expected. Can mean the DPF is missing, has a crack, there's an exhaust leak, or the pressure sensor hoses are blocked. Requires live data investigation.
DPF Differential Pressure Too High (Bank 1)
Pressure is higher than the ECU expects — a classic sign of a blocked DPF. The filter is restricting exhaust flow. Often logged when the blockage is too severe for a forced regeneration to work.
DPF Pressure Sensor Circuit Fault
A fault in the differential pressure sensor itself — not necessarily a blocked DPF. The sensor or its hoses may be faulty, giving the ECU false readings. Replacement of the sensor may resolve this without any DPF clean needed.
DPF Regeneration Duration Too Long
Regeneration is taking longer than the ECU expects. This suggests the DPF has a high soot load or there's a fault preventing efficient regen — such as a failing injector or EGT sensor issue.
DPF Restriction — Soot Accumulation
A generic code indicating the DPF is restricted due to soot build-up. Very commonly searched. Usually means a professional clean is required — but root cause diagnosis must happen first to prevent repeat blockage.
Exhaust Gas Temperature Sensor Fault
EGT sensors before and after the DPF monitor temperatures during regen. A fault here can abort regeneration prematurely — leading to a DPF that never fully cleans itself even on long drives.
Book a Diagnostic — From £45
PATS / Eolys Fluid — The Warning People Miss
Peugeot, Citroën, Ford, and Volvo vehicles built on PSA platforms use an additive system to help the DPF regenerate. When the fluid runs out, a scary "Engine Fault" message appears. It's actually a straightforward fix.
What Is the Eolys / PATS System?
These vehicles have a secondary tank that automatically injects a small amount of Eolys fluid into the fuel. This additive lowers the temperature at which soot ignites, making DPF regeneration more effective — especially on slower roads.
Around 100,000 miles (or sooner if never topped up), the tank runs dry. The ECU logs permanent fault codes and warning messages appear. The car may enter reduced power mode.
The fix is refilling the tank with OE-grade Eolys fluid and resetting the additive counter in the ECU. Topping up without the ECU reset will not clear the warning.
```Common PATS Fault Codes
Our PATS / Eolys Service Includes:
- Refill with OE-grade Eolys fluid
- ECU additive counter reset
- DPF saturation check
- All fault codes cleared on completion
- Professional diagnostic tooling throughout
- Most vehicles completed same day
Prices from £120 inc VAT. Confirmed on booking based on make and model.
Which Cars Use This System?
Dashboard Messages to Look For:
Seeing one of these? We can usually fix this same day.
Book PATS / Eolys Service ```AdBlue System Diagnosis & Repairs
AdBlue (Diesel Exhaust Fluid) is used by most post-2015 diesel vehicles to reduce harmful NOx emissions. Faults with the system can prevent your car from starting — and need specialist diagnosis.
AdBlue Top-Up Service
Running low? We refill your AdBlue tank with quality DEF fluid and reset the warning. Quick, affordable, and often same day.
AdBlue Pump Failure
The dosing pump can fail over time — often crystallising due to contamination. We diagnose and replace with photo evidence of the fault.
Injector & Nozzle Faults
The SCR injector delivers AdBlue into the exhaust stream. Blockages or failures cause NOx faults and persistent warnings. We clean or replace as needed.
NOx Sensor Faults
Faulty NOx sensors trigger AdBlue warnings even when fluid level is fine. We test and replace using specialist diagnostics — not just a code clear.
SCR Catalyst Issues
The SCR catalyst converts harmful NOx gases using AdBlue. If contaminated or degraded — often by wrong fluid being added — the whole system can fail.
Frozen AdBlue Tank
AdBlue freezes at around -11°C. Most vehicles have heated tanks, but heater failures in cold weather cause temporary faults. We diagnose whether it's a freeze or a genuine failure.
Common AdBlue Fault Codes
SCR NOx Catalyst Efficiency Below Threshold
The SCR system isn't reducing NOx sufficiently. Could be a depleted AdBlue tank, contaminated fluid, or a failing catalyst.
Reductant Quality Too Low / Wrong Fluid
Wrong fluid has been added, or the AdBlue has degraded. Engine may count down remaining starts before refusing to start.
Reductant Pressure Too Low
AdBlue pump isn't building enough pressure. Usually indicates a pump failure or blockage in the dosing system.
Reductant Level Sensor / Level Low
Tank level is critically low, or the level sensor is faulty. Acting immediately prevents the vehicle from being locked out.
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Keep Your DPF Healthy
The best DPF repair is the one you never need. Simple driving habits can prevent the most common cause of blockage.
Regular Motorway Runs
Drive at 50–70mph for 20–30 minutes at least once a fortnight. This allows the DPF to complete a passive regeneration cycle naturally and burn off accumulated soot.
Always Use Low-SAPS Oil
Every service must use the correct Low-SAPS oil (typically 5W30 C3 spec). Standard oil accelerates ash deposits that cannot be regenerated away — only physically removed.
Fix Warning Lights Promptly
Any warning light — especially glow plug or engine management — should be diagnosed quickly. Unresolved faults almost always lead to DPF problems down the line.
Don't Skip Servicing
Worn injectors, dirty fuel filters, and old oil all cause excess soot production. Regular servicing keeps combustion clean and DPF soot levels manageable throughout the year.
Ford Fuel Vaporiser Valve Faults
Many Ford diesel models — including the Transit, Focus, Mondeo, Kuga, and Galaxy — use a fuel vaporiser (also called a hydrocarbon injector or HC injector) fitted in the exhaust system. It's a common failure point that's often misdiagnosed as a blocked DPF.
What Does the Fuel Vaporiser Do?
During active DPF regeneration, the ECU needs to raise exhaust temperatures to burn off accumulated soot. To do this, it sprays a fine mist of fuel through the vaporiser valve directly into the exhaust stream, which ignites and creates the heat needed for a successful regen cycle.
When the vaporiser fails — usually because it blocks, sticks open, or the solenoid burns out — the car can no longer complete an active regeneration. The DPF fills with soot, the warning light comes on, and the car eventually goes into limp mode. The DPF itself may be fine.
This is why a proper diagnostic is so important — cleaning the DPF without testing the vaporiser system will result in almost immediate re-blockage.
```Symptoms of a Failing Vaporiser
What We Do
We diagnose the full Ford DPF regeneration system — including the vaporiser valve, temperature sensors, and regen history — before recommending any clean. If the vaporiser is faulty, we replace it and verify correct regen operation before the job is complete.
Book a DiagnosisAdBlue System Faults Beyond the Obvious
Sometimes the AdBlue or SCR system has failed beyond what a top-up or sensor replacement can fix. In certain situations — particularly for off-road, agricultural, or motorsport applications — software-based solutions may be the answer. We're upfront about what's legal and what isn't.
When Physical Repair Isn't the Answer
In some cases — particularly older vehicles, commercial vehicles with high repair costs, or vehicles used exclusively off-road — a genuine SCR catalyst failure or damaged AdBlue pump system can cost more to repair than the vehicle is worth.
We can diagnose the full SCR system and give you an honest assessment of your options. Where a manufacturer-level software or calibration solution exists to address an SCR fault, our remap partners may be able to help.
Our Remap & Tuning Services
Through our remap and tuning solutions page, we offer a full range of ECU calibration work — including performance remaps, EGR solutions, DPF software calibration, and diagnostic tuning for vehicles experiencing emissions system issues in eligible use cases.
- ✓Performance remaps for diesel vehicles
- ✓EGR & DPF calibration (off-road / eligible use)
- ✓AdBlue / SCR system diagnostics & options
- ✓DPF pressure & sensor recalibration
Your DPF Questions Answered
Got a DPF Problem?
Let's Sort It Properly.
Diagnosis first. Fixed prices. Photo evidence. No waffle. Stevenage's 5-star rated independent DPF specialists — RAC Approved.
