The California Air Resources Board (CARB) has admitted that the normal and predicted operations of the diesel particulate filter (DPF) damages heavy-duty diesel engines.

In a dramatic reversal of its previous position, CARB explains that the potential for serious engine malfunction and damage arises as the DPF device ages on the engine. When the DPF becomes clogged with soot, it must go through repeating cycles of regeneration to burn off the soot. Over time, operating exactly as intended, the buildup of soot in the DPF and extremely high heat damages the filter and other components of the engine, and this, in turn, creates excessive heat buildup and backpressure in the engine, which eventually and inevitably causes engine damage. All of this is set forth in CARB’s Notice of Public Hearing for April 22, 2016, and supporting documentation, proposing to make aftermarket parts available for the repair of the 2007-2009 DPFs, including retrofitted and original equipment manufactured (OEM) DPFs.

In CARB’s own words:

“[T]he trapped soot in the wall-flow DPF builds up over time, increasing the backpressure in the engine as it continues to operate. Operating the engine at excessive backpressure for extended periods will impact engine performance and eventually cause engine damage.”

CARB further states:

“As these engines age, there is an increasing need to replace the OEM DPFs as the parts experience wear. Engine problems such as faulty turbochargers, bad fuel injectors, or malfunctioning EGR [Exhaust Gas Recirculation] valves can negatively impact DPFs in several ways including catalytic poisoning, fouling or overloading the DPF, or causing thermal damage due to more frequent regeneration.”

CARB now admits the destructive effect of the extreme heat during the regeneration process, which is a necessary operation of the DPF. CARB concedes that the DPF device will break down over time, and that it will cause engine damage.

This has been the long-held position of the Alliance for California Business (the Alliance). In 2013, the Alliance filed a lawsuit challenging the legality of CARB’s regulations that require truck and bus owners to have DPF devices on their vehicles as a condition of operating in California, because the DPF, operating exactly as designed, is mechanically unreliable and dangerous. The Alliance told CARB, through declarations of truck owners, operators, mechanics, and an expert in the field of automotive engineering, that the DPF disables and damages engines as well as engine components through the repeated exposure to extremely high temperatures and backpressure within and near the engine compartment. The Alliance presented evidence that these conditions have led to explosive truck and bus fires, and caused brush fires and damage to nearby property. CARB blamed the DPF problems on poor maintenance (blaming the owners and operators) and has steadfastly refused to consider, much less investigate these fires.

Now CARB admits that the DPF’s filter component disintegrates over time because it cannot withstand the extremely high temperature that is necessary for the DPF to operate, and along the way it damages other parts of the engine. But it gets worse – CARB proposes to fix the problem by facilitating the creation of yet another line of dangerous and defectively designed DPF replacement parts – to be paid for, once again, by California businesses.

It is time for CARB to do the right thing: issue a recall on all DPFs until they are proven safe and mechanically reliable.

The Alliance remains committed to defending California truck and bus owners and operators, and all those who rely on trucks and buses for the transportation of goods and people, from this regulatory debacle.

For further information, please contact:

Bud Caldwell, President
Alliance for California Business Telephone: 530-342-6511
Website at:

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