P2002

Powertrain
Manufacturer-specific
Emissions

Severity

Diagnose soon

The engine is not in mechanical danger and the vehicle usually drives normally. But the vehicle will fail emissions testing, some applications derate power, and a substrate that melted or cracked almost always did so because of an upstream problem that will destroy the replacement filter too.

Safe to drive: Generally drivable if power feels normal. If it derates or enters limp mode, get it in before it strands you somewhere inconvenient.

Standard definition

Diesel Particulate Filter Efficiency Below Threshold (Bank 1)

In plain English

The engine computer measured how much the diesel particulate filter restricts exhaust flow and found too little restriction. Exhaust is passing through more freely than a working filter allows, which points to a filter that is cracked, hollowed out, or missing, or to a pressure sensor reporting wrongly.

This is the opposite condition from a clogged filter. The module compares the pressure drop measured across the filter by the differential pressure sensor against the pressure drop it expects for the current exhaust mass flow and temperature. When measured restriction falls well below the model, the filtering substrate is presumed compromised. Some newer applications add a downstream particulate matter sensor that directly detects soot slipping past the filter and can set this code on that evidence instead.

How the car detected it

During qualifying steady operating conditions, usually a sustained cruise with exhaust flow high enough to produce a meaningful signal, the module samples differential pressure and compares it against a modelled expectation derived from mass air flow, fuel rate, and exhaust gas temperature. Efficiency below the calibrated threshold across the evaluation window sets the code. Because it needs a specific drive pattern, the monitor may not run for many miles after a code clear.

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Can I keep driving?

Safe to drive: Generally drivable if power feels normal. If it derates or enters limp mode, get it in before it strands you somewhere inconvenient.
Urgency: Diagnose soon
Will it pass emissions: No
If ignored: A compromised filter is no longer trapping particulate, so the vehicle is emitting soot it was certified not to emit and will fail inspection. More practically, whatever damaged the substrate is usually still present. A dribbling injector, a failed exhaust temperature sensor, or a runaway regeneration event will cook the next filter the same way, and diesel particulate filters are one of the most expensive emissions components on the vehicle. Some manufacturers also progressively reduce power once this fault is confirmed.
Check engine light: Solid
Clearing the light: A completed drive cycle is normally required before the monitor re-runs and the light can clear.

Symptoms you may notice

Check engine light or DPF warning light on
Power loss / limp mode
Increased fuel consumption
Failed forced regeneration attempts

Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.

Likely causes

These are the causes this code can have. For the order to work through them in, see what to check first.

#	Cause	How often	Component
1	DPF heavily loaded — repeated short trips prevent regeneration	Very common	—
2	Filter substrate cracked or melted from runaway regeneration	Common	—
3	Differential pressure sensor drift	Common	—
4	Leak around DPF housing	Occasional	—

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

What to check first

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

Whether the filter is physically still there and still full of substrate
On a used vehicle, a gutted or deleted filter is a genuine and common finding. It costs nothing to look and it ends the diagnosis immediately.
The differential pressure sensor hoses
These small hoses crack, melt against hot components, plug with soot, and occasionally get reconnected backwards during unrelated work. Any of those mimics a hollowed-out filter perfectly and the repair is trivial.
The differential pressure reading with the engine off
With no exhaust flow, both ports see the same pressure and the sensor should report essentially zero difference. A nonzero reading at rest means the sensor has drifted, and this takes seconds to check.
Every other stored code
Injector, exhaust gas recirculation, boost, exhaust temperature, or fuel system codes stored alongside this one usually name the reason the filter failed. Fixing the filter without fixing them wastes the new filter.
The exhaust joints between the two pressure taps
A leaking clamp, gasket, or cracked housing between the sampling points lowers measured restriction without the filter being damaged at all. It is a visual and audible check.

How a shop diagnoses this

Read all codes and full freeze frame before clearing anything
Tools Scan tool with manufacturer-level diesel data
Capture engine speed, load, exhaust temperatures, and calculated soot and ash loading at the time of the fault. Note whether any fault that would inhibit or corrupt the monitor was also present.
Expected
A full picture of what the module believed at the moment it made the call, plus any companion faults that explain it.
Physically verify the filter and its substrate
Tools Inspection light, borescope, basic hand tools
Inspect the filter assembly. Where a sensor port or the filter inlet is accessible, look at the substrate directly. On a used or unfamiliar vehicle, confirm it has not been gutted or replaced with an empty shell.
Expected
Intact ceramic substrate with uniform, unmelted channels and no visible crack or hole through the centre.
Remove, inspect, and clear the pressure sensor hoses
Tools Hand tools, compressed air
Detach both hoses, check for soot plugging, cracking, melting, and correct routing to the correct ports. Blow them clear and reconnect to the correct taps.
Expected
Both hoses open, flexible, undamaged, and connected to the correct upstream and downstream ports.
Check the sensor zero point with the engine stopped
Tools Scan tool live data
Key on, engine off, read the differential pressure value. Compare against a second reading after the sensor has been disconnected from the hoses and left open to atmosphere on both ports.
Expected
Essentially zero differential pressure with no flow. A meaningful offset indicates a drifted sensor.
Log differential pressure against exhaust flow while running
Tools Scan tool with data logging
With the engine at idle and then at elevated speed under load, record differential pressure alongside mass air flow and exhaust temperature. The signature of a compromised substrate is very low pressure drop despite high flow and a soot load the module believes is significant.
Expected
Pressure drop that rises meaningfully with flow. A flat, near-zero curve at high flow supports a compromised filter.
Review regeneration and loading history
Tools Manufacturer-level scan tool
Read calculated soot mass, ash mass, distance since last successful regeneration, and the count of completed versus aborted regenerations. A history of extreme regeneration temperatures or a runaway event explains a melted substrate.
Expected
A coherent history. Very low soot mass alongside high mileage supports a filter that is no longer trapping anything.
Pressure or smoke test the exhaust between the sampling points
Tools Exhaust smoke machine, soapy water
Introduce smoke or low-pressure air and look for leaks at clamps, gaskets, welds, and the filter housing itself. Include the sensor tap fittings.
Expected
No escaping smoke between the upstream and downstream pressure taps.
Identify and repair the upstream cause before replacing anything
Tools Injector return test equipment, scan tool, factory service information
If the substrate is genuinely damaged, establish why. Look at injector return volume and balance rates, exhaust gas recirculation function, exhaust temperature sensor accuracy, turbocharger condition, and oil consumption. Melted substrates come from excess fuel reaching the filter.
Expected
A named upstream cause, or documented evidence that all upstream systems are functioning correctly.

Possible repairs and what they cost

Repair	Only after you confirm	Typical cost	Who
Clear or replace the differential pressure sensor hoses and correct their routing	Hoses found plugged, cracked, melted, or connected to the wrong ports	$30–$180	DIY
Replace the differential pressure sensor	Sensor reports a nonzero differential with no exhaust flow, or fails to respond to a known applied pressure	$120–$400	DIY
Repair an exhaust leak between the upstream and downstream pressure taps	Smoke or pressure test shows escaping gas between the two sampling points	$120–$500	DIY
Professional off-vehicle filter cleaning	Substrate confirmed intact on inspection but loaded with non-combustible ash rather than damaged	$300–$700	Shop
Replace the diesel particulate filter assembly	Substrate visually cracked, melted, or hollowed, or differential pressure stays near zero at high exhaust flow with a verified good sensor and no exhaust leaks	$1,500–$5,000	Shop
Repair the upstream fault that damaged the filter	Injector return, exhaust gas recirculation, turbocharger, or exhaust temperature testing identifies a specific out-of-specification component	$400–$3,000	Shop

Filter prices swing enormously between a light-duty passenger diesel and a heavy-duty truck, and between original equipment and aftermarket. Note also that removing or disabling a diesel particulate filter is illegal on road vehicles in the United States, so delete kits are not a repair option regardless of price.

Common mistakes with this code

Replacing the filter before checking the two pressure sensor hoses. That check takes minutes and frequently ends the job.
Confusing this code with a clogged filter and running forced regenerations. A cracked substrate will never pass, no matter how many regenerations you force.
Replacing the filter without finding what melted it, which guarantees the replacement fails the same way.
Fitting a cheap aftermarket filter with different flow characteristics than the module's model expects, which can set the code again on a brand-new part.
Treating an emissions-only code as ignorable. It is an automatic inspection failure and on some vehicles it triggers a power derate.
Assuming a low soot reading is good news. On this code, a suspiciously low soot load is part of the evidence against the filter.

Components involved

Diesel Particulate Filter
Dpf Differential Pressure Sensor
Dpf Temperature Sensor

Sources

Definition: Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
Diagnostic guidance: Written and reviewed in-house. See our editorial policy.
Review status: Reviewed

Found something wrong? Tell us — we publish corrections.

Whether the filter is physically still there and still full of substrate

The differential pressure sensor hoses

The differential pressure reading with the engine off

Every other stored code

The exhaust joints between the two pressure taps

Read all codes and full freeze frame before clearing anything

Physically verify the filter and its substrate

Remove, inspect, and clear the pressure sensor hoses

Check the sensor zero point with the engine stopped

Log differential pressure against exhaust flow while running

Review regeneration and loading history

Pressure or smoke test the exhaust between the sampling points

Identify and repair the upstream cause before replacing anything