P246C – Diesel Particulate Filter Restriction – Forced Limited Power

Reinier

By Reinier (Contact Me)
Last Updated 2020-12-20
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P246C Diesel Particulate Filter Restriction - Forced Limited Power -

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What Does Code P246C Mean?

OBD II fault code P246C is a generic trouble code that is defined as “Diesel Particulate Filter Restriction – Forced Limited Power”, and is set when the PCM (Powertrain Control Module) detects a condition in which the diesel particulate filter is either clogged or restricted to the point where the exhaust backpressure reaches unacceptable levels.

Note that this code only affects diesel vehicles that are fitted with diesel particulate filters.

While diesel exhaust after-treatment technology has developed to the point where gaseous diesel emissions are approaching the low levels of gasoline emissions, these technologies cannot remove solid particulate matter, also known as “soot” from diesel exhaust.

Thus, to address the problem of solid soot particles (that can be as fine as solid particles in cigarette smoke) in diesel exhaust, the latest iteration of exhaust emissions regulations requires that car manufacturers fit devices known as DPF (Diesel Particulate Filters) to their diesel vehicles. In practice, these devices are required to physically trap and contain accumulated soot particles until they can be oxidized, or burnt off through one of several processes.

These oxidizing processes are known as “regeneration” processes, but we need not delve into the complexities of these processes here, beyond saying that when a regeneration process completes successfully, the filter medium in the DPF is cleared of accumulated soot and ash to “regenerate” it to the point where it can again begin to trap and contain soot particles. As a practical matter, though, while DPF regeneration processes happen automatically, whether (or not) these processes are initiated is based on factors like vehicle usage, fuel quality, driving style, and the type of regeneration process in use on the vehicle.*

*Automatic  regeneration processes can be active, or passive. In passive processes, regeneration is initiated by the exhaust gas temperature as a consequence of normal driving, while in active processes, regeneration is initiated either by changes to injection timing to increase the exhaust gas temperature or by the injection of raw fuel (or other additives) into the DPF to create the temperature required to burn off accumulated soot.

In terms of practicalities, the PCM monitors the exhaust backpressure continuously via dedicated exhaust pressure and temperature sensors to determine or calculate the extent of the soot accumulation in the DPF. On most diesel vehicles, the normal exhaust backpressure is typically around four to five pounds per square inch, and the PCM uses this value as the starting point to calculate the soot load in the DPF. As a practical matter, the exhaust backpressure rises in direct proportion to the soot load: the higher the soot load, the higher the exhaust backpressure becomes, which the PCM interprets as decreasing DPF efficiency.

In most applications, the PCM is programmed not to allow the DPF efficiency to decrease below about 85% (or higher on some applications) before initiating a DPF regeneration process. In a fully functional system, and provided all required pre-conditions are met, the process can take anywhere from about ten minutes to about twenty minutes to complete successfully. However, in some cases, such as vehicles that are primarily used in city driving, the exhaust gas never reaches the required temperature to initiate or complete a regeneration event and in these cases, the DPF can become clogged in short order.

Regardless of the mechanism of DPF regeneration, though, if any fault, failure, or defect occurs that prevents successful DPF regeneration, and thereby causes the exhaust pressure to rise to above acceptable or maximum allowable limits, the PCM will recognize that the DPF is clogged or defective, and it will set code P246C and illuminate a dedicated warning light as a result.

Note that in some cases, the PCM may also initiate a limp mode in which engine power is severely restricted or limited when this code sets to protect the engine and other exhaust components. This condition will typically persist until the fault is found and corrected. Note that while a forced regeneration event performed with a suitable scan tool might resolve the issue, there is no guarantee that forced regenerations will always resolve limp modes.

Where is the P246C sensor located?

This image shows the basic operating principles of a typical DPF device. Note, however, that many DPF devices resemble other exhaust components such as catalytic converters and mufflers, so be sure to consult reliable service information for the affected application to avoid confusion, misdiagnoses, and the possible unnecessary replacement of expensive exhaust parts and/or components.

What are the common causes of code P246C?

Common causes of code P146C are largely similar across all applications, and could include one or more of the following-

  • Failed, clogged, or damaged DPF substrate
  • Defective or failed exhaust backpressure sensor(s)
  • Use of “rebuilt” or substandard aftermarket DPF devices
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the exhaust after-treatment system
  • Repeated failed regeneration events
  • Repeated failures to initiate regeneration events caused by improper (albeit normal for the vehicle owner) vehicle usage
  • Excessive oil consumption caused by mechanical engine issues
  • Poor fuel quality, most notably the use of high concentrations of biodiesel (Typically, blends than contain more than 5% of biodiesel)

WARNING: Attempts to correct or resolve this and similar codes by removing or bypassing the DPF are viewed by authorities as tampering with the emissions control system and are therefore illegal.

What are the symptoms of code P246C?

Common symptoms are largely similar across all applications but note that depending on the nature of the problem, the severity of some symptoms listed here could vary between applications-

  • Stored trouble code and illuminated warning light
  • The vehicle may be locked into a limp or fail-safe mode
  • Depending on the nature of the problem, multiple seemingly unrelated codes could be present along with P246C
  • Idling quality may be poor or the idling speed may fluctuate
  • Fuel consumption may increase noticeably
  • In some cases, a no-start condition may be present as a result of the fail-safe mode
  • If the vehicle is drivable, severe power loss may be present
  • The engine may run hotter than normal, and in severe cases, the engine may overheat

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