Preventive maintenance (PM) is defined as a fundamental, planned maintenance activity designed to improve equipment life and avoid any unplanned maintenance activity. It incorporates systematic inspection, detection, correction, and prevention of incipient failures.

Nothing in this definition says PM is a static, never-changing activity that does not require training or process updates once technicians are fully trained or the makeup of the fleet does not change.

PM processes, procedures, and technician training must be continually adjusted, updated, and modified with the latest technology.

In 2010, diesel truck operators in the U.S. were introduced to selective catalytic reduction (SCR) aftertreatment systems in Class 2 through Class 8. This introduction added hardware, software, significant operational changes, and high costs to any fleet operating SCR-equipped trucks.

Given the added complexity and significance of the changes wrought by SCR’s introduction, fleet operators should consider these changes by asking, “has our preventive maintenance process been amended to include aftertreatment inspections?”

Diesel trucks, from light to heavy, have undergone significant technological and mechanical changes in recent years; nowhere is this more evident than in the evolution of exhaust system aftertreatment. The impact on maintenance costs resulting from SCR system adoption has been remarkable. Fleets report maintenance cost increases of $0.08 to $0.10 per mile and, in other cases, aftertreatment system maintenance now rivals tire costs as their most expensive maintenance item.

In addition to driving up capital cost by several thousand dollars, the impact of SCR is the “gift that keeps on giving” by significantly increasing maintenance costs.

What many fleet operators have yet to recognize is that, while the SCR aftertreatment system may become a truck’s most expensive maintenance item, it is likely the most vulnerable to compromise and failure due to seemingly minor and unrelated causes that until now may have been largely ignored in routine maintenance practices.

While all diesel-powered trucks built since 2007 are impacted, and especially those built in 2010, the most vulnerable to premature SCR failure are those in the vocational segment. Why? Because SCR’s best operational objective yielding the most favorable maintenance outcome is high temperature for a sustained period. Vocational trucks offer neither. They seldom reach the necessary temperature for a long enough duration to do the system any good.

Consequently, aftertreatment operational problems and total system failures are more common in vocational truck applications. There are steps that can be taken by vocational fleet operators to mitigate this failure rate and they begin with adopting preventive maintenance processes specifically geared toward recognizing, preventing, and correcting conditions both upstream and downstream that result in compromising the system, leading to expensive failures.

Adding these steps to the current inspection process will pay dividends by reducing downtime and costs.

Engaging Passive Regen

Before the vehicle enters the shop and before the PM inspection begins, perform a passive regeneration (regen). While the truck is waiting to enter the shop, perhaps while parked on the deadline, develop a routine that includes the performance of a regen.

Completing a regen will ensure that when the PM inspection begins, the exhaust and aftertreatment system is in the best condition possible and the on-board diagnostic (OBD) system will hopefully recognize this by adjusting the fault codes and malfunction indicator lights (MILs) accordingly, providing the technician with the most accurate information on the current state of the system. Otherwise it’s likely, especially with a vocational truck, the technician will learn a regen is required and must delay the PM to complete the regen before continuing the inspection.

For safety and operational reasons, allow the system to cool before beginning the inspection.

Thorough Visual Inspection

As impressive as an SCR aftertreatment system appears visually, the technology, engineering, and science behind the system also makes it extremely sensitive to vibration, dirt, and grime as well as to upstream irregularities that may have been ignored on trucks not so equipped.

By incorporating a thorough visual inspection of the system to include cleaning the exposed contacts and wiring, will pay dividends. Cracks and breakage due to vibration may not be visibly evident without a 360-degree visual inspection using proper lighting and a hands-on inspection procedure.

There are several sensors and connectors located on aftertreatment devices and components, including a soot sensor, which manufacturers added in 2016. This device was added to alert maintenance personnel to a diesel particulate filter (DPF) fault code indicating excess soot or ash present in the exhaust stream.

Upstream Jeopardy

As vulnerable to truck operational characteristics as these systems truly are, they are even more vulnerable to mechanical anomalies that might not have been immediately corrected in the past. For instance:

• Exhaust Leaks: Prior to SCR introduction, repairing an exhaust manifold leak might have been postponed until it could no longer be ignored. On today’s trucks, an exhaust leak may prevent the engine and subsequently the aftertreatment system from achieving the desired temperature for maximum effectiveness resulting in premature system failure. When detected, any exhaust leaks should be repaired regardless of how minor.

• Coolant Leaks: Unlike an exhaust manifold leak, a coolant leak would likely be addressed when detected. Many maintenance professionals may not know that a coolant leak can have immediate and detrimental impact on the aftertreatment system. Because such leaks can occur in numerous locations, including air or exhaust gas recirculation (EGR) coolers, PM processes should be modified to include inspection procedures that assure detection of water intrusion in all possible areas. When detected, coolant leaks must be addressed; they can easily result in fatal cracking within the aftertreatment system, resulting in expensive repairs and lengthy downtime.

• Excessive Idling: For some fleets, excessive idling is an operational necessity. For others, idling is an operational nightmare. In either case, idling, especially if it’s excessive, will be a primary cause for aftertreatment failure by prematurely plugging the DPF. A telematics system will offer idling history and, while not specifically a PM process, reviewing that history routinely can serve as an alert to this condition and trigger correction.

• Mechanical Issues: Oil leaks, leaking fuel injectors, and failure of the turbocharger will all have detrimental effects on the aftertreatment system. Anything that can introduce unburned oil, excessive ash, rust, dirt, or other debris into the downstream exhaust system will have negative impacts. Strong PM inspection processes will detect many of these conditions which should be addressed and repaired at that time.

• Fuel Quality: Water is present in every gallon of diesel fuel; this is yet another operational fact. High-quality fuel and diesel exhaust fluid (DEF) are essential for aftertreatment system longevity. Routine PM processes, such as replacing diesel fuel filters and regularly servicing water separators, should remain a priority. When practicing extended oil drain intervals, fuel filter, and water separator servicing should continue to be performed at the OEM-prescribed intervals regardless, even when oil drains fall outside those parameters.

The learning curve associated with aftertreatment care, maintenance, and service has been steep and expensive, and the industry is still learning. Fleet recognition of the delicate nature of this large and expensive component system followed by the adoption of PM processes designed to inspect, detect, and correct problems may prevent more expensive failures and maximize the capabilities of their aftertreatment systems.