Heading off the diesel aftertreatment demons – don’t let fear of high costs distract you from basic maintenance

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Updated Jun 11, 2024

Let’s talk about exhaust aftertreatment demons. They scream at you in the form of warning lights on the dash or, worse, a repair bill you weren’t expecting well into the four figures and, sometimes, five figures. Other times that bill is spent on nothing more than diagnostics as days turn into weeks with recurring issues mechanics are routinely stumped by.

Hope for an affordable solution fades with each recurrence – dash lights in the form of the upside-down triangle, a laptop screen filled with codes that are difficult to understand. The lost days, the stress of truck payments, growing credit card bills … all of it is enough to make an owner-operator doubt his or her choices. It wasn’t supposed to be like this, right? You purchased a newer used truck – still under warranty, the safety net you needed while traversing the high-wire risk of ownership. Right?

Meet John Osinga of Lynchburg, Virginia, a successful and financially stable trucking business owner since 1980. In 2019, John did just that, investing in a 2015 model with 377,000 miles on the odometer and a brand-new diesel particulate filter. This was to be an upgrade from his 2003 Kenworth T2000 after 1,700,000 miles.

Newer truck, he hoped, fewer issues.

The trouble started in the second half of 2020 as the rig approached 500,000 miles. First with the dash warnings. Then, when he took the rig in for maintenance, various sensor replacements.

He got to something of a “final straw” point as he was exiting a customer’s facility and, at the guard shack, the de-rate warning lit up, meaning at best John could drive a short distance at 5 mph before everything shut down entirely. John returned the loaded trailer to the yard, called to arrange for a repower, and had the truck towed to the last shop that had worked on the aftertreatment system.

They once again performed a computer diagnostic check, and what was really needed was a video scope of the internal parts to look for the cause. At last, they discovered more than one issue. John had the DPF cleaned and replaced, and it didn’t appear the be in the proper position (something some mechanics note can happen when excess back-pressure builds up). Additionally, they discovered that a diffuser plate that the diesel exhaust fluid doser sprays against was broken and had fallen out of place, causing the DEF not to atomize into the fine particles needed to work properly. The cause was unknown.  Osinga and I speculate it could have been corrosion from the DEF itself.

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Before discovering this issue, Osinga, upon advice, had replaced the outlet NOx sensor twice, then a DEF pump and doser valve, also the inlet NOx sensor. Emphasizing some techs’ willingness to just throw parts at a problem, he noted, “sometimes I think relying solely on the computer and the tests available stops people from looking further into what the issue is” at its core. “We need to ask what’s causing the failure.”

In some cases, might one root cause be the very high cost of aftertreatment maintenance and filter replacement itself?

The more issues Osinga experienced and the more costs mounted, he began to feel as if replacing or rebuilding the whole aftertreatment system might have worked out better financially for him. Yet with the investment already put into the system, such a dramatic move was now not in the cards.

clean me drum barrelRead more about DPF ash cleaning via this link to a round-up of past maintenance-related coverage in Overdrive. Emissions-system-related stories are under the "Trailers and other topics" header.

I talked to longtime independent service shop owner Jeff Gray with Gray’s Garage in Pontiac, Illinois, for some thoughts independent of the engine and truck manufacturers. Gray and his employees’ years of successful front-line experience suggest behaviors by drivers and owners can contribute to growing aftertreatment problems. These are things you have a high degree of control over, no doubt:

  • It may all begin with how you start your truck. When you turn on the key, allow the ECM to power up the whole system and read all the sensors before actually cranking the truck. And if your truck has idle automatic shutoff? Be sure to turn off the key and begin the cycle over again.
  • The manufacturer may have updated programing for the computer system – ask questions of any service manager you encounter during maintenance or contact the manufacturer directly.
  • If there is black soot in the exhaust pipe, either the DPF is bad or was previously bad and has been replaced – it should be inspected.
  • If the engine seems to crank over slowly (suggesting a likely low voltage), after the engine has run enough to charge the batteries, restart the engine (low voltage tends to be the underlying reason for a number of codes).
  • Be careful you don’t void your warranty by delaying repairs.
  • If the temperature is below freezing, condensation can freeze inside your aftertreatment system and result in fault codes. To try and solve this you can increase the RPMs or force a regen to build up heat, then turn off your engine. Wait at least one minute, then begin again by turning on your key, waiting for the ECM to read all sensors, then crank the truck to see if the codes clear. 

gary buchs and jeff gray at Gray's Garage shop in Pontiac, IllinoisJeff Gray (right) with yours truly at his Gray's Garage shop in Pontiac, Ill.To help reduce these problems, the No. 1 thing to keep high in the mind is the service interval for the DPF itself. It has been a moving target from engine manufacturers, though generally with successive generations the interval seems to get longer and longer. Once damage starts, though, there's also a compounding effect throughout various components of aftertreatment systems, Gray emphasized. Thus it may make some sense to have the DPF cleaned or replaced on a schedule that's a degree quicker than what's recommended for an over-the-road rig. (Don't forget changing the DEF filter, too – most recommendations appear to be to do that on an annual basis.)

Talking to owners, I find the majority avoid maintaining these filters, relying simply on the truck’s own warning and regen systems to tell them what to do.

Osinga hit on one of the major disincentives for owners: “Do you know how much it costs for this filter?” That cost can be a deterrent to thinking about a filter replacement, in other words, but for a smart owner it ought not to be a deterrent to appropriate preventive maintenance.

I unscientifically surveyed my small-business truck group and asked how much each owner spent on maintenance and repairs to their aftertreatment system over half a million miles, when Osinga's began to break down. Costs seemed to range between about 1 and 4 cents a mile over the period, or between $5,000 and $20,000. Budget for at that level of outlay in your planning if you’re investing in a newer-model used truck. Two-three years down the road, you’ll be better prepared for the hit when it comes. 

ICYMI: Two months following the publication of this story, the author, Gary Buchs, joined Overdrive editor Todd Dills, Jeff Gray, and Pittsburgh Power's Bruce Mallinson for a roundtable discussion of diesel emissions maintenance issues, heavily centered around getting the right diagnosis the first time. View that discussion here: 

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