Emissions, exhaust and fuel system
Catalyst, DPF, EVAP, AdBlue and everything the emissions test looks at. ← Browse sections
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Bang on lift-off: exhaust backfire and what it means
A pop from the exhaust when you lift off is either an aftermarket calibration doing it deliberately or unburnt fuel reaching a hot catalyst. Those are very different.
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Euro 7 and on-board monitoring: what changes for owners
The next emissions standard adds continuous self-measurement, brake and tyre particulates, and battery durability. Here is what actually reaches the driver.
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Exhaust smell inside the cabin: treat it as urgent
Exhaust gas contains carbon monoxide, which is colourless, odourless and disabling. Smelling exhaust in the cabin means something is letting it in.
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P204F: reductant system performance and the AdBlue you cannot ignore
The dosing system is not delivering what it should. Crystallised urea is the usual reason, and short journeys are the usual reason for that.
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P20EE: SCR NOx catalyst efficiency below threshold
The SCR system is not converting enough NOx. That has several possible causes, and only one of them is the catalyst itself.
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P242F: DPF restriction from ash accumulation — the filter that cannot be saved
Soot burns away during regeneration. Ash does not. This code is the point at which no amount of driving or forced regeneration will help.
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Real driving emissions: why lab figures never matched your car
The gap between official figures and reality was not a rounding error. RDE testing exists because the old laboratory cycle could be engineered around.
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Revs hang between shifts: emissions calibration, not a fault
Lift off to change gear and the revs stay up for a moment before falling. Almost every modern manual does it, and the reason is not what most people assume.
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Surging at a constant speed: fuel trim, torque converter or EGR
A gentle rhythmic pulsing at a steady cruise, with your foot still. The frequency of the pulse points at which system is hunting.
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Ticking at idle: injectors, lifters or an exhaust leak
Some ticking is entirely normal and some means an engine is running out of oil pressure. The difference is in how the noise responds to temperature and revs.
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AdBlue warning countdown: what happens when it reaches zero
The countdown is not a suggestion. At zero the engine will not restart, and that behaviour is written into emissions regulation rather than chosen by the manufacturer.
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DPF light on: the drive that clears it and the one that does not
The filter needs about twenty minutes of sustained motorway speed to burn off soot. Most people give it ten minutes of town driving and wonder why the light stays.
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Fuel gauge reading wrong: sender, float or instrument cluster
A gauge that sticks, jumps or reads empty with a full tank is nearly always the sender inside the tank. How it misbehaves tells you which part of the sender failed.
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Rattling over bumps: how to find the loose part
Rattles are frustrating because they only happen while you are driving. A systematic bounce test in the driveway finds most of them in twenty minutes.
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Exhaust modifications: what breaks besides the sound
Removing a catalyst or particulate filter changes more than the noise. The consequences for sensors, codes, and the annual test.
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The fuel gauge lies: sender, float, or module
A needle that sticks or jumps almost always means the sender in the tank. The symptoms and why range estimates go wrong too.
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Fuel economy myths: seven tips that do not work
Some popular economy advice achieves nothing and some works in reverse. Seven of the most common, with the mechanics explained.
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Rattling underneath: heat shield, mounting, or silencer
A metallic rattle at particular revs usually costs almost nothing to fix. How to tell a heat shield from a blown silencer, and why not to run it on.
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The fuel filter: symptoms of a blockage and why diesel is a special case
A blocked filter shows up under load and at high revs. The symptoms, and why on a diesel the question is more expensive.
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Bad fuel and misfuelling: what to do and what not to do
Filled up with the wrong fuel? The rule is do not start it. The symptoms of bad fuel, the difference between petrol in a diesel and the reverse, and what diagnostics shows.
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Diesel or petrol: an honest comparison by cost of ownership, not advertising
The question is decided by your route, not by fuel consumption. A diesel on short urban trips costs more to own than a petrol — and that is exactly where the fuel saving falls apart.
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Choosing engine oil: why the approval matters more than viscosity or brand
A bottle carries three groups of markings and they are not equivalent. Viscosity says how the oil flows. The approval says whether it suits your engine. Confusing them costs a particulate filter or a cam phaser.
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HVO diesel: what this fuel is and whether you can put it in your engine
Pumps increasingly carry an HVO or XTL marking. It looks like diesel, costs differently, and behaves slightly differently in an engine. What it is, who it suits, and what changes in the data.
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Buying a car with LPG: what to check besides the gas equipment itself
Everyone inspects the gas system. Few check what it did to the engine over a hundred thousand kilometres — and that is where the difference between a good buy and an expensive repair lives.
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Fuel additives: which ones work and which ones sell hope
The additive shelf promises everything: clean injectors, raise octane, revive the catalyst. A breakdown by category — what has a physical mechanism, what works narrowly, and what cannot work at all.
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Wrong fuel in the tank: what to do in the first minutes to avoid paying for the fuel system
The cost of the mistake hinges on one decision — whether to start the engine. Both scenarios, why petrol in a diesel and diesel in a petrol are different orders of severity, and what diagnostics shows.
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Air conditioning smell: where it comes from and why the spray can does not fix it
That musty smell when the fan starts is biofilm on the evaporator. Why it grows there, why aerosols buy you two weeks, and one free habit that solves it permanently.
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Bad fuel: how to recognise it over OBD and tell it apart from a real fault
After a fill-up the car pulls worse and stumbles under load. Is it the fuel or did something break? The data answers unambiguously — through knock correction behaviour, fuel trims, and when the codes appeared.
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Fuel consumption is up: telling normal variation from a real fault in ten minutes
Consumption depends on season, driving style, and route — and those three explain up to 30 % of the difference. How to filter out normal variation, and the three parameters that answer the question.
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Fuel smell in the cabin: when it is trivial and when you stop immediately
A fuel smell inside the car is the one symptom worth taking seriously straight away. Where it comes from, which variants are harmless, and how OBD narrows the source in ten minutes.
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Petrol particulate filters: why a GPF clogs in city traffic and what to do about it
Particulate filters have a reputation as a diesel problem. Since 2017 they have been fitted to direct-injection petrol cars too — and they clog the same way: short trips, city driving, an engine that never gets hot.
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Secondary air injection P0410: runs 90 seconds a day and fails anyway
A pump that only runs on a cold start, and only for a minute and a half. It sits idle the rest of the time — which is precisely why it corrodes, seizes, and eventually produces a code that fails your emissions test.
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AdBlue and SCR System OBD-II Diagnostics: P20EE, P229F, P2BAD on Euro 6 Diesels
Common OBD-II fault codes for AdBlue/SCR systems on Euro 6 diesels: P20EE NOx catalyst efficiency, P229F DEF quality, P2BAD urea concentration — causes, Live Data, solutions.
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Catalytic Converter Theft: OBD-II Detection and Prevention Guide
How to detect catalytic converter theft via OBD-II: P0420/P0430 after theft, misfires, exhaust noise — plus practical prevention, marking, and insurance advice.
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LPG and CNG Cars: OBD-II Diagnostics for Gas Fuel Systems
How OBD-II helps diagnose LPG (autogas) and CNG vehicles: P0093 gas pressure, P0172 mixture, lambda integration, petrol-to-gas switching — practical OBD guide.
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Oxygen Sensor (Lambda Probe) OBD-II Diagnostics: P0130–P0167 and Live Data
DTC codes P0130, P0131, P0134, P0136, P0141 — oxygen sensor diagnosis via OBD-II Live Data: wideband vs narrowband, STFT/LTFT correlation, identifying the faulty bank without an oscilloscope.
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EGR valve and EGR cooler leak: codes P0401, P0402, P0404 and why diesel Passat/A6 "got sooted up"
How to diagnose EGR via OBD-II: difference between P0401 (low flow) and P0402 (high), EGR cooler leak, why "EGR delete" is a short fix with long problems.
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DPF (Diesel Particulate Filter): diagnosing regeneration, clogging and when it's time for a clean
What a DPF is, how it regenerates, which DTCs signal a problem (P2002, P244A, P244B), how to read soot levels via OBD-II, and why short trips kill diesel engines.
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Fuel Trim (STFT and LTFT): how the fuel correction tells you about engine health
What Short-Term (STFT) and Long-Term (LTFT) Fuel Trim mean, which values are normal, and how to diagnose a vacuum leak, dirty MAF or clogged injector from Fuel Trim numbers alone.
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O2 sensor waveform: how to tell a dead lambda from a live one by shape
Visual analysis of upstream and downstream O2 sensor signals: normal oscillations, response signs, aging indicators, how to distinguish a "flat" curve from a real problem.
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Diesel Engine Power Loss: 10 Causes and Step-by-Step Diagnosis
Why a diesel car does not pull, how to quickly distinguish turbo, EGR, DPF or injector issues, and which DTC codes to read first.
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Exhaust Smoke: White, Blue, Black — What Each Colour Means
Decoding tailpipe smoke colour: how to tell in 30 seconds whether it is condensation, oil burning, rich mixture or a blown head gasket.
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Fuel Consumption Went Up: 12 Causes and How to Diagnose via OBD-II
Sudden fuel-use increase: which DTC codes and live parameters to watch, how to distinguish worn engine from a clogged filter.