P2002: Particulate Filter Efficiency Below Threshold — Bank 1
What to do
Do not accept a filter replacement as the first step. Ask whether a forced regeneration was attempted and what the pressure differential was afterwards — that single measurement distinguishes a filter that is merely full from one that is finished, and they cost very different amounts.
If most of your driving is short and local, that is very likely the cause rather than a fault. A weekly run of half an hour at sustained speed lets the filter clean itself, and it is worth building into your routine whatever the outcome here. If the car ever displays a message asking you to keep driving, do exactly that — it is mid-regeneration, and switching off partway is what turns a self-clearing filter into a blocked one.
What this code means
The diesel particulate filter is not doing its job to the standard the module expects. This is one of the most expensively misdiagnosed codes on any modern diesel, and the reason is that a blocked filter and a failed filter present identically while costing completely different amounts.
How the filter is supposed to work. It traps soot, and periodically burns it off — a regeneration — by raising exhaust temperature to around six hundred degrees for several minutes. That requires sustained load and a hot exhaust. On a motorway it happens unnoticed. In town it may never happen at all.
Which produces the single most common cause: a filter that is simply full. A car used for short cold journeys accumulates soot faster than it can burn it, the filter loads up, and eventually the module reports it as ineffective. Nothing has failed. A forced regeneration on a scan tool, or in milder cases a sustained motorway run, restores it — and a filter replaced without addressing the usage will block again on exactly the same schedule.
Ash is the case that genuinely does need replacement. Soot burns away; the incombustible ash left behind by engine oil additives does not, and it accumulates permanently over the filter's life. A filter full of ash rather than soot will not respond to regeneration, and that distinction is measurable — the differential pressure across it stays high after a successful regeneration.
And a large share of these are caused by something else entirely. A filter only blocks abnormally fast if the engine is producing abnormal soot: a leaking injector, a failing turbo, a stuck EGR valve or a blocked air filter. Fitting a filter without finding that cause replaces the symptom.
The order that saves money: verify the pressure sensor, attempt a forced regeneration, measure the pressure differential afterwards, and find what is making the soot before condemning the filter.
Likely causes
Start here
- Predominantly short journeys · easy
- Incomplete particulate filter regeneration · moderate
- Faulty exhaust gas differential pressure sensor · moderate
- Leaking fuel injector · moderate
- Stuck EGR valve · moderate
Less common
- Ash loading at end of filter life · specialist
- Faulty turbocharger · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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- The car — make, model, year and engine. The same fault behaves differently on each.
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