OBD-II trouble code
P2002: Diesel Particulate Filter Efficiency Below Threshold Bank 1
The engine controller has calculated that the diesel particulate filter is no longer trapping soot the way it should. "Efficiency" here is a maths result derived from pressure and temperature readings — which is why a cheap sensor hose can produce a quote for a very expensive filter.
Quick facts
- System
- Powertrain
- Category
- Exhaust / Aftertreatment
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $60 – $3,500
- DIY difficulty
- Shop recommended
What does P2002 mean?
A diesel particulate filter is a ceramic honeycomb in the exhaust with alternate channels plugged, forcing exhaust gas through porous walls that catch soot. It fills up by design. The controller periodically burns the accumulated soot off by raising exhaust temperature — a regeneration — and the filter returns to a low restriction. P2002 says that cycle is no longer producing the result it should on bank 1.
The critical thing to understand is that the controller cannot see inside the filter. It infers filter condition, almost always from a differential pressure sensor measuring the drop across the filter, cross-referenced against exhaust temperature sensors and a soot-loading model. "Efficiency below threshold" is therefore the output of a calculation, not a direct observation. Any input to that calculation that goes wrong produces this code with a perfectly healthy filter in the exhaust — and the differential pressure sensor's two small hoses, which run from the exhaust into a plastic sensor body, are notorious for soot-plugging, cracking at the fitting, or melting where they pass near the pipe. A hose costing a few pounds and a filter costing thousands present identically on a scan tool.
The second distinction that matters is soot versus ash. Regeneration burns soot, which is carbon, back out as gas. It does not remove ash — the incombustible residue from engine oil additives and normal wear, which stays in the filter permanently and slowly consumes its capacity. A filter that has covered a high mileage can be genuinely at the end of its life with nothing broken anywhere, and no number of forced regenerations will recover it. That is why the diagnosis has to establish which situation the vehicle is in before money is spent.
The third is driving pattern, and it is the most common real-world cause. Regeneration needs sustained exhaust temperature, which needs sustained load. A diesel used for short urban trips may never complete a regeneration, so each attempt is aborted part-way and soot accumulates faster than it is burned. The vehicle is not faulty. It is being used in a way its aftertreatment was not designed for, and the fix is partly a conversation about how the vehicle is driven.
Common causes
- Differential pressure sensor hose soot-blocked, cracked, melted or disconnected
- Failed differential pressure sensor reporting an implausible drop across the filter
- Filter genuinely loaded with ash at high mileage and at the end of its service life
- Repeated incomplete regenerations caused by short-trip, low-load driving
- Exhaust leak upstream of or between the pressure sensor tapping points
- Faulty exhaust gas temperature sensor preventing regeneration from being commanded or completed
- Excessive soot production from a fuel injector, turbocharger or EGR fault loading the filter faster than it can be cleared
- Engine oil consumption depositing ash far faster than normal
- Aftermarket or removed filter internals, or a poor-quality replacement filter
Symptoms
- Check engine light on, often with a separate DPF or exhaust filter warning lamp
- Reduced power or limp mode once soot loading passes a limit
- Regeneration attempts happening far more often than normal, or a regeneration warning that never clears
- Noticeably worse fuel economy
- Cooling fans running hard and a hot smell after driving
- Rough idle or a change in exhaust note
- Visible smoke under acceleration
- Engine oil level slowly rising on vehicles that regenerate by post-injection
Diagnostic steps
- 1.Read the live soot load, ash load, differential pressure and exhaust gas temperature values before touching a spanner. Those four numbers together will usually tell you whether the filter is loaded, whether it is worn out, or whether the sensor is lying.
- 2.Check the differential pressure reading with the engine off. It should sit at or very near zero. Any meaningful offset with no exhaust flow means the sensor or its hoses are the fault, not the filter — this is a thirty-second test that has saved a great many unnecessary filter replacements.
- 3.Remove and inspect both pressure sensor hoses. Look for soot packing, cracks at the fittings, chafe and heat damage. Blow them through and refit.
- 4.Compare the recorded ash load against the vehicle's mileage. A high ash figure on a high-mileage filter is a genuine end-of-life finding and no service will reverse it.
- 5.Review the regeneration history: how many attempts, how many completed, how long since the last successful one. A pattern of aborted regenerations points at driving style, at a temperature sensor, or at something interrupting the cycle rather than at the filter.
- 6.Check for and repair any exhaust leak between the sensor tapping points before condemning anything, because a leak lowers the measured pressure drop and corrupts every reading downstream of it.
- 7.Look for an upstream cause of excessive soot — injector condition, turbo boost control, EGR operation, oil consumption. Fitting a new filter behind an engine that is still overloading it buys a short reprieve at high cost.
- 8.If everything checks out and the filter is genuinely loaded with soot rather than ash, attempt a forced regeneration with a capable scan tool and re-evaluate afterwards.
Repair cost
$60 – $3,500
The spread on this code is enormous and that is the point. A pressure sensor hose is $10 to $60 and a differential pressure sensor $80 to $300 fitted. A forced regeneration at a shop is $100 to $250. Off-vehicle filter cleaning, where the filter is sound but ash-loaded, is $300 to $700 and is worth asking about. A replacement filter is $1,000 to $3,500 depending on the platform, and on some vehicles considerably more. Never approve a filter replacement without seeing the pressure sensor and hoses ruled out first.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with diesel particulate filter (dpf) cleaning / replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.