|Trouble Code||Fault Location||Probable Cause|
|P2459||Diesel Particulate Filter Regeneration Frequency||DEF fluid, DPF system failure, Wiring, Sensors|
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What Does Code P2459 Mean?
OBD II fault code P2459 is a generic code that is defined as “Diesel Particulate Filter Regeneration Frequency”, and is set on diesel vehicles when the PCM (Powertrain Control Module) detects that the regeneration frequency of the DPF (Diesel Particulate Filter) is not conforming to a predefined and/or specified rate.
While technologies such as common rail fuel injection and selective catalytic reduction converters have greatly reduced the volume of diesel exhaust emissions that cause smog in urban areas, these technologies cannot eliminate or remove solid particulate matter in diesel exhaust. Commonly known as “soot”, the particulate matter consists of un-combusted hydrocarbon particles that are typically finer than the particles in cigarette smoke.
As a practical matter, the solid particles in diesel exhaust can only trapped in a filter, and later burned off when the soot load in the filter exceeds a critical level, and to do this effectively, engineers and chemists have developed an effective filter that can do just that. In terms of operation, a typical DPF device contains a very fine substrate through which the exhaust gas can flow relatively freely. However, since the device is designed to capture and contain fine particles, the soot will collect and accumulate in the filter until the soot starts to clog up the substrate, raising the pressure in the exhaust system, which is the underlying mechanism that triggers DPF regeneration events.
In terms of operating principles, the PCM uses dedicated pressure sensors to monitor the exhaust backpressure as a means to determine the soot load in the DPF. Thus, when the PCM deems the soot load to be excessive (based on the exhaust backpressure) it will begin a process of regenerating the DPF in one of two ways, depending on the vehicle. One way involves burning off the accumulated soot by raising the exhaust gas temperature, which is typically accomplished by altering the fuel injection timing to allow some combustion to take place in the exhaust manifold.
The other method involves injecting a precisely metered amount of a urea and water mixture (aka ADBlue, or Diesel Exhaust Fluid) into the exhaust system just before the DPF. In a fully functional system, the urea is converted into ammonia, which then acts as a catalyst that initiates the oxidation of the accumulated soot by raising the internal temperature of the DPF to a level where the soot is effectively consumed by the high temperature.
While both systems work very well, the frequency of the rate at which the DPF is regenerated depends both on the way the vehicle is driven, as well as accurate input data from multiple sensors, some of which include exhaust gas temperature sensors, exhaust gas pressure sensors, dedicated NOx sensors, and others. Provided that all inputs are accurate, plausible, and continuous, the PCM will initiate DPF regeneration whenever it deems the soot load to be excessive, without any inputs from the driver.
However, on systems that use a liquid reductant, the regeneration frequency also depends on accurate input data from multiple sensors that monitor the entire reductant injection system. Typical monitoring includes the fluid level in the storage tank, its temperature, the integrity of the injection pump’s wiring and control system(s), and the operation of the reductant metering valve and injection nozzle.
One other factor that comes into play is the mileage driven between regeneration events. While this distance is not fixed, it is factored into both the way the vehicle is driven, and the operating conditions that obtained since the last regeneration event took place. In practice though, all of the requirement and/or enabling conditions described above have to be met, or fulfilled for the PCM to be able to maintain the regeneration frequency of the DPF.
Thus, when a failure or malfunction exists that prevents the frequent regeneration of the DPF the PCM will set code P2459, illuminate one or more warning lights, and may also initiate a limp mode and/or disable the starting circuit, depending on the vehicle and the exact nature of the problem.
Where is the P2459 sensor located?
The image above is a generic representation of a typical diesel exhaust system that uses a liquid reductant to initiate a regeneration event. Note that while exhaust system design differences exist between vehicle makes and models, diesel exhaust systems typically consist of all the components and sensors shown in this image, although the actual location(s) of some components may differ somewhat from the exhaust system layout shown here.
What are the common causes of code P2459?
The causes of code P2459, and indeed, all other DPF related codes are many and varied, but could include one or more of the following-
- Low DEF fluid levels
- Dirty, or contaminated DEF fluid
- Almost any fault, defect, malfunction, or failure of any component and/or sub-system in the reductant injection system has the potential to both affect the operation of the DPF, and the frequency at which the DPF is regenerated. Note though that in cases where additional codes are present along with P2459 (some of which may be manufacturer specific), the stored codes must be resolved in the order in which they were set to avoid a misdiagnosis
- Damaged DPF substrate: note overheating of the DPF almost invariably destroys the DPF device
- One or more defective exhaust sensors: these could include exhaust gas temperature sensors, exhaust pressure sensors (including differential pressure sensors), NOx sensors, and others that may include pressure and flow sensors that are related to the liquid reductant injection system
- Damaged, burnt, shorted, disconnected, and/or corroded wiring and electrical connectors anywhere in the exhaust after treatment system
- In cases where the DPF regeneration is accomplished with hot exhaust gas, as opposed to injecting a liquid reductant, repeated short trips such as occurs in the urban driving environment, the exhaust gas may never reach the temperature required to initiate a regeneration event. In these circumstances, the soot load in the DPF may accumulate to the point where the PCM initiates a failsafe or limp mode, or disables the starting circuit to protect the engine and other exhaust system components
What are the symptoms of code P2459?
Common symptoms of code P2459 could include one or more of the following, but note that some symptoms of code P2459 and other DPF related codes could immobilize the affected vehicle until the root cause of the code(s) is resolved-
- Stored trouble code(s)and illuminated warning light(s)
- No start condition: note that all DPF regeneration systems that use a liquid reductant incorporate a counter that calculates the number of engine starts that can be made with the volume of reductant in the storage tank. Based on vehicle usage, the PCM will disable the starting circuit when it is likely that the reductant will run out during the current trip. However, while the PCM will allow the vehicle to complete the current trip, it will initiate a failsafe or limp mode during the last trip, and prevent the vehicle from starting again until either the fault is corrected, or the DEF fluid storage tank is refilled.
- In cases where the DPF regeneration is accomplished with hot exhaust gas, as opposed to injecting a liquid reductant, the vehicle may experience a degree of power loss. Depending on the vehicle, the PCM may initiate a failsafe or limp mode or disable the starting circuits when the soot load in the DPOF exceeds a maximum allowable threshold.