P24A4 – Diesel Particulate Filter Soot Accumulation Excessive

Reinier

By Reinier (Contact Me)
Last Updated 2022-07-11
Automobile Repair Shop Owner

CodeFault LocationProbable Cause
P24A4 Diesel Particulate Filter Soot Accumulation Excessive
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What Does Code P24A4 Mean?

OBD II fault code P24A4 is a generic trouble code that is defined by most carmakers as “Particulate Filter Restriction Soot Accumulation Too High Bank 1 and is set when the PCM (Powertrain Control Module) detects an excessively high soot load in the DPF (Diesel Particulate Filter) associated with the exhaust system that serves Bank 1. Note that “Bank 1” refers to the bank of cylinders that contains cylinder #1.

NOTE: An exception to this definition is a variation used by carmaker Jeep, which defines this code as “Particulate Filter Restriction- Soot Accumulation Too High”. In practice, the slightly different wording conveys the same meaning as the original ISO/SAE- authorized definition, but in terms of implementation, the variation used by Jeep is typically (but not always) applied to vehicles that only have a single DPF.

While greatly improved engine designs, common rail fuel injection technologies, and highly capable fuel control strategies have produced the cleanest diesel engines yet, these advanced technologies are unable to address the issue of diesel soot, which is the visible black “smoke” that results from diesel combustion.

Although this black smoke has largely disappeared from modern diesel exhaust, it has only become invisible because there is so little of it emitted by individual diesel vehicles. However, the combined emissions of diesel soot by millions of diesel vehicles amount to several hundred thousand tons of soot particles being released into the atmosphere every day, and one of diesel soot’s most harmful effects is that it can, and does cause severe respiratory ailments in humans.

To address this problem, all manufacturers of diesel vehicles are now legally mandated to fit devices, commonly known as DPFs (Diesel Particulate Filters) to their products that burn diesel fuel. These devices are described in various quality standards, and at a minimum, a compliant DPF must be able to trap and contain all diesel soot particles in the exhaust stream until a predetermined soot load is reached. At this point, the PCM will initiate one of several strategies, (depending on the vehicle make and model) to oxidize (burn off) the accumulated soot in a process known as a DPF regeneration. This simply means that the soot load is burned off the restore the DPF’s ability to accumulate, trap, and contain a new load of soot until the next regeneration process is initiated.

We need not delve into the complexities of the various regeneration strategies here, but suffice to say that these processes are controlled by one or more control modules, and occur without any input from a driver. In practice, however, there are too many possible reasons why a DPF regeneration could fail to initiate to list all, or even most here, but in all cases, the effect of such failures is that the soot load in the DPF eventually reaches a point where the DPF becomes clogged, restricted, or otherwise unable to trap any more soot.

In terms of operation, almost all DPF systems use two exhaust pressure sensors, one on each side of the DPF to monitor the pressure in the exhaust system. As a practical matter, the exhaust pressure will progressively increase on the intake side of the DPF as the soot load in the DPF increases, while exhaust pressure will remain relatively constant at the outlet side of the DPF.

Since the PCM monitors these pressure values continuously, it uses the pressure differential between the two sensors to a) infer the volume of the soot load in the DPF, and b) calculate an efficiency value for the DPF based on the inferred soot load. In fully functional DPF systems, the PCM will typically not allow the DPFs efficiency to fall below about 85 percent before initiating a regeneration event.

As mentioned elsewhere, regeneration events can and do fail for a wide variety of reasons, but in the context of code P24A4, the PCM only considers the soot load, and by extension, the DPFs calculated efficiency, when determining whether (or not, as the case may be) to set the code and illuminate a warning light.

Thus, regardless of the possible causes of an excessive soot load, the PCM will set code P24A4 and illuminate a warning light when the affected  DPFs efficiency falls below a predetermined threshold. In addition, the PCM may also restrict the engine’s power output, as well as initiate and maintain no-start condition until the fault is found and corrected.

Where is the P24A4 sensor located?

This schematic diagram shows the location (circled) of the DPF device relative to other major exhaust system components on several VAG-group applications, but note that this layout is not representative of all VAG-group applications or of any other vehicle make and model. As a practical matter, the location of a DPF device on any given vehicle is as much determined by the regeneration system in use on the vehicle, as it is by the design of the vehicle.

It should be noted that regardless of the location of eh DPF device on any vehicle, these components are not user-serviceable, and replacing a DPF device require special tools and equipment, as well as a high level of technical ability. Therefore, we do NOT recommend that non-professional mechanics attempt to remove, service, clean, or replace a DPF device on any vehicle. Thus, if you suspect that you are dealing with a defective DPF device, the best course of action would be to seek professional assistance with diagnosis and repair.

What are the common causes of code P24A4?

The most common causes of code P24A4 are not only many and varied; they are also dependent on whether the vehicle uses a passive or active DPF regeneration system. Therefore, we will list possible causes of code P24A4 in two sections, starting with causes relating to passive systems-

NOTE: Passive regeneration systems use strategies like delayed fuel injection and/or variable valve timing to cause some fuel to combust in the exhaust manifold, thus creating the required temperature to “light off” the regeneration process in the DPF. Typical causes of failed DPF regeneration events with passive systems could include one or more of the following-

  • Defective or corrupted fuel injection drivers in the PCM; this condition typically causes a failure to establish combustion of fuel in the exhaust manifold
  • Failures, defects, or malfunctions in the VVT (Variable Valve Timing) system, which also fail to establish combustion of fuel in the exhaust manifold
  • Broken or damaged filter media in the DPF
  • Damaged, defective, or malfunctioning exhaust pressure sensor(s)
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors anywhere in the DPF system

NOTE: Active regeneration systems use either a liquid reductant (a solution of water and urea) to initiate the regeneration process or additional raw fuel that is injected into the DPF to create the required heat to initiate the regeneration process. Typical causes of code P24A4 could include one or more, or several of the following-

  • Contaminated or degraded liquid reductant, but note that the only reliable remedy for this condition is the (hugely expensive) replacement of the entire reductant injection system
  • Depletion of the liquid reductant: in these cases, the PCM will allow the trip to continue until the engine is switched off, but it (the PCM) will initiate and maintain a no-start condition immediately after the engine is switched off. Simple replenishment of the reductant will usually (but not always) resolve this condition, depending on the vehicle make and model
  • Any fault, failure, or malfunction of any part or component of the liquid reductant injection system that affects the correct operation of the injection in any way, but note that diagnosing this system typically requires the use of advanced diagnostic equipment
  • Failures, defects, or malfunctions of the system that injects additional raw fuel into the DPF
  • Damaged, defective, or malfunctioning exhaust pressure sensor(s)
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors anywhere in the DPF system
  • Broken or damaged filter media in the DPF

What are the symptoms of code P24A4?

While the typical symptoms of code P24A4 largely depend on the type of DPF regeneration system the vehicle uses, the most common symptoms of this code could include one or more of the following-

  • Stored trouble code and one or more illuminated warning lights, depending on the regeneration system
  • Multiple exhaust after treatment-related codes could be present along with P24A4
  • Multiple codes relating to excessively high exhaust gas temperatures and/or exhaust backpressure could be present
  • A no-start condition could be present that will persist until the fault is found and corrected
  • Varying degrees of power loss could be present at some or all engine speeds/loads
  • Fuel consumption may increase significantly, depending on both the regeneration system used and the nature of the problem
  • In some cases, repeated DPF failures may occur if the underlying cause of the problem remains unresolved
  • Serious engine damage may occur if excessively high exhaust back pressures are allowed to continue for extended periods
  • In some cases, it may be impossible to initiate a forced regeneration procedure with a scan tool
  • The affected vehicle will not pass a mandatory emissions test