What Are the Symptoms of a Defective Diesel Particulate Filter?

Although failed (and failing) DPFs can and do cause serious symptoms like no-start conditions and others that can include the immobilization of an affected vehicle, most DPFs fail progressively over extended periods, and therefore, symptoms appear gradually- in fact, sudden failures of DPF’s are relatively rare.

Nonetheless, the most common symptoms of failed, failing, or defective DPFs are largely similar across all diesel vehicle makes and models, although the severity of some symptoms may vary between different vehicle makes and models. Below are some details of the most common symptoms of DPF failures-

Stored trouble codes and illuminated MIL lamp

All OBD II systems are programmed to turn on the MIL (Malfunction Indicator Lamp) on the dashboard when it detects any condition, such as a defective DPF, that has the potential to affect exhaust emissions negatively. In addition, the OBD II system will also set and store appropriate trouble codes that relate to the efficiency of the DPF.

Reduced engine performance

When a DPF works as designed, the exhaust stream passes through the device with no or very little resistance. However, when the DPF is blocked or even partially obstructed by accumulated diesel soot of carbonized engine oil, the free flow of the exhaust gas is impeded, which raises the pressure in the exhaust system to beyond acceptable levels.

When this happens, the exhaust system cannot scavenge, or “pull” exhaust gas from the cylinders efficiently, which means that a part of the subsequent air/fuel mixture charge is displaced by residual exhaust gas. As a practical matter, this means one of two things: the first is that the reduced volumes of the air/fuel charge translate into reduced engine power, and the second is that larger throttle openings to increase the engine’s power output cannot compensate for the reduced engine power.

Hard, or no-start conditions

All internal combustion engines depend on the free flow of exhaust gas through the exhaust system to start and run. However, although a slightly blocked DPF might allow some exhaust gas to flow through it, thereby allowing the engine to start and run, albeit poorly, a severely blocked DPF could prevent any exhaust gas from flowing through it. In such a case, the engine might start, but it will not continue to run for more than a few seconds because the exhaust system cannot remove exhaust gas from the cylinders for longer than a few seconds after the engine has started.

Reduced fuel efficiency

The natural tendency of most drivers is to use larger throttle openings to compensate for reduced engine power, but this only results in more fuel being injected into the cylinders without a commensurate increase in engine performance.

Since the exhaust system cannot remove exhaust gas from the cylinders efficiently when excessive exhaust back pressures are present, the increased volumes of fuel being injected produce more unburned fuel, which greatly increases the volume of soot in the already blocked DPF. Essentially, this creates a kind of vicious circle scenario; injecting more fuel into the cylinders leads to even poorer combustion, which creates more soot, which blocks the DPF even more, which in turn, increases the exhaust back pressure even more, which further reduces the exhaust system’s ability to remove exhaust gas from the cylinders even more.

In terms of practicalities, this vicious circle always results in fuel economy that gets progressively worse as the DPF becomes progressively more clogged up with soot.

Engine overheating

Although the liquid-based cooling system performs the bulk of the work involved in regulating the engine’s temperature, the efficient scavenging of exhaust gas from the cylinders plays a critical role in preventing the engine’s temperature from rising to unacceptable levels.

Therefore, any condition, such as a blocked or clogged DPF that inhibits the scavenging of exhaust gas from the cylinders can cause the engine’s temperature to rise to undesirable and potentially dangerous levels.

Turbocharger damage

Although turbochargers are designed to endure very high temperatures for extended periods without suffering adverse effects, excessively high engine temperatures caused by a blocked DPF can (and often does)  cause serious degradation of the engine oil that lubricates some o the rotating parts in the turbocharger.

It is important to note that since the volume of oil that moves through the turbocharger bearings and other rotating parts at any given moment is relatively small, excessive temperatures in the turbocharger cause the oil to carbonize, thus clogging up the small-diameter oil passages in the turbocharger. When restrictions in the oil passages caused by carbonized oil reach and/or exceed a critical point, the lubrication of all the rotating parts in the turbocharger fails, which always results in severe, if not always fatal damage to the turbocharger.

A strong smell of exhaust gas may be present

Since clogged-up or restricted DPF devices always cause excessive exhaust back pressures to develop, some exhaust gas might escape through joints or junctions in the exhaust system. It should be noted that this could potentially be deadly dangerous since some exhaust gas might enter the passenger cabin through poorly sealed joints and drain hole plugs in the vehicle's floor.

Other places where exhaust gas could enter the vehicle include poorly sealed doors and/or windows, through forced ventilation ducting, and through the A/C system's drain tube that typically extends some way below the level of the vehicle's floor pan.

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