P2002: diesel particulate filter efficiency low

P2002 means the diesel particulate filter is not filtering as effectively as it should, or is clogged with soot. It is most common on diesels used mainly for short journeys, which never get hot enough for long enough to burn the soot off. Diagnose soon and drive gently. Indicative UK repair cost is £100 to £1,500.

A particulate filter is designed to fill up and then empty itself. It traps soot, and periodically the engine raises the exhaust temperature enough to burn that soot away, which is the regeneration you may have read about. Regeneration needs sustained heat, and sustained heat needs a sustained run. A car doing ten minutes to the shops and ten minutes back never finishes the job it keeps starting, and the soot simply accumulates until something has to give.

Can I drive with P2002?

Diagnose soon, drive gently. If the car is still driving normally and has not gone into limp mode, the most useful thing you can do costs nothing: take it for a proper run of twenty minutes or more at steady motorway speed, in a slightly lower gear than you would normally choose, so the engine is working and the exhaust is hot. That is precisely what the car has been asking for. If it is already in limp mode, or the soot load has gone past the point where it will attempt a regeneration at all, then further driving only adds to the problem and it needs a garage.

What P2002 means for your MOT

This is the one code in this library where the MOT consequences can be more serious than the repair, so it is worth being exact. Everything below is a major defect in the DVSA MOT inspection manual.

  • The filter as a component. Section 8.2.2.1 instructs the tester to inspect visible, identifiable emissions components and names diesel particulate filters explicitly. A filter that is missing, obviously modified or obviously defective fails.
  • Tampering, as a separate defect. The manual lists "evidence that the diesel particulate filter has been tampered with" on its own, and says a filter that has clearly been cut open and rewelded should be rejected unless the person presenting the vehicle can show there was a legitimate reason, such as professional cleaning. If a car has had its filter removed, that is a failure, and a remap does not hide an empty casing from someone looking underneath it.
  • The lamp. Section 8.2.2.2, for diesels first used on or after 1 July 2008.
  • Visible smoke. A filter that is damaged or has been bypassed lets soot straight through, and clearly visible black smoke is assessed by eye as its own defect.

What usually causes it

  1. A clogged filter. Soot has gone in faster than the car has managed to burn it off. A forced regeneration, or an off-car clean, very often recovers the filter, and both are a long way cheaper than replacing it.
  2. A faulty differential pressure sensor, or its pipes. The car calculates how full the filter is from the pressure drop across it. The thin pipes feeding that sensor block with soot or split with age, and then the car reports a filter problem that is not there.
  3. Regenerations that keep being interrupted. Either by journeys that end too soon, or by another stored fault. Many cars will not start a regeneration at all while an EGR, turbo or injection fault is logged, so this code can be a symptom of something else entirely.

What it costs to fix in the UK

£100 to £1,500. A forced regeneration on a garage's diagnostic equipment is the bottom, and so is a pressure sensor with new pipes. An off-car chemical clean sits in the middle. A replacement filter is the top, and aftermarket filters vary enormously in how long they last.

One thing worth knowing before you authorise anything: a filter at the genuine end of its life is full of ash rather than soot, and ash does not burn off. No amount of motorway driving and no forced regeneration recovers that one, which is why reading the soot load matters before money is spent. If you are weighing up a garage visit, what a car diagnostic costs in the UK puts the investigation fee in context against those figures.

These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.

What to check first

  1. Look at the fuel gauge and the oil. Plenty of cars refuse to begin a regeneration below roughly a quarter of a tank. And a diesel that has suffered repeated failed regenerations can end up with fuel diluting its engine oil, which is a separate problem that needs dealing with on its own terms.
  2. Read the soot load, the differential pressure and the distance since the last successful regeneration. Those three figures together tell you whether you are looking at a genuinely full filter or a sensor telling fibs, and they cost nothing but the time to read them.
  3. Read every other stored code before touching the filter. If there is an EGR, boost or injection fault logged, deal with that first. The car may well be declining to regenerate because of it.
  4. Only then attempt a regeneration. A long run if the car is still willing, or a forced regeneration on diagnostic equipment if it is not. Replacing the filter is the last step, not the first.

Related codes

Get it narrowed down before you spend

A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.

Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.