The late-model diesels in our record set share a headline complaint, and it is not the engine. It is the emissions hardware bolted to it. EGR, DPF and SCR faults dominate the failure evidence on every chassis and engine family where we have a real record, Cummins and Detroit and Volvo and Mack alike. That much is documented and we will stand behind it.
What we will not do is hand you a tidy price table. Go looking for aftertreatment repair costs and you will find dozens of pages quoting confident dollar ranges for a DPF, an EGR cooler, an SCR catalyst. Almost none of them say where the number came from. We checked what we could actually source, and the honest answer is that some of these parts we can price to the cent and some of them we cannot price at all.
What the three systems actually do
EGR is exhaust gas recirculation. It routes a share of burned exhaust back into the intake to lower combustion temperature, which lowers NOx. The side effect is soot and carbon going somewhere it was never going to be welcome.
The DPF is the diesel particulate filter. It traps soot, and periodically the truck burns that soot off in a regeneration cycle. Ash, unlike soot, does not burn off. It accumulates, and the filter eventually has to come off the truck and be cleaned.
SCR is selective catalytic reduction. It injects diesel exhaust fluid into the exhaust stream through a doser valve to convert NOx, with NOx sensors watching the result.
Three systems, three separate ways to end up on the side of the road.
The one complete aftertreatment invoice we have
Here is the most useful number on this page, because it is a real bill from a real breakdown, read out on camera by the owner who paid it.
A Mack Anthem with an MP8 lost its turbo and actuator with DPF fouling. The total came to $9,136.54. The breakdown: labor $837.50, parts approximately $5,787, DPF filter cleaning and shipping $312.50, car rental $105, and four nights in a hotel.
Read that line-item list again, because it is the part the price-range articles never show you. The parts and the wrench time are only one slice of that bill. The rest was the cost of being a person who is not at home and not moving freight. Four nights of hotel and a rental car are not a footnote on an owner-operator's aftertreatment failure. They are the shape of it. And that figure does not include a single dollar of revenue the truck did not earn while it sat.
EGR: the one component we can actually price
A Detroit DD15 EGR valve, part A4701402108, was listed at $1,350.00 at dieselrebuildkits.com on a 2026-08-19 price check. That lines up with a mechanic in our video record putting the EGR valve at roughly $1,000 or more in parts plus two or more hours of labor.
Two independent sources, one listed price and one working shop, landing in the same place. That is about as solid as parts pricing gets in this business.
EGR coolers: we do not have a price, and here is why
We went looking for an EGR cooler price and came back with nothing. The DD15 presenter in our record has not had to replace a cooler on his own engine, which he describes as luck rather than durability, and no cooler price was captured anywhere else in the set.
We know coolers fail. One mechanic documents EGR coolers leaking coolant straight into the combustion chamber. A Volvo D13 source describes the EGR system failing three ways: the EGR leaks, or the cooler clogs, or you get an EGR differential-pressure fault. The failure mode is well documented. The repair bill is not. If you see a specific EGR cooler price online, ask where it came from, because we could not find one.
DPF: one real number, and one honest gap
The only DPF figure we own is $312.50, which is what the Mack owner paid to send his filter out for cleaning and get it back. That is a cleaning, not a replacement.
We do not have a sourced DPF replacement price. Not for a Cummins, not for a Detroit, not for a Mack. We could estimate one. We are not going to, because somebody would price a truck off it.
What we do have is the interval that matters more than the price anyway. DPF ash cleaning is needed roughly every 250,000 to 400,000 miles. When you are looking at a truck with 400,000 miles on it and no ash-cleaning record, that is not a maintenance question. That is a bill sitting in the near future with no date on it yet.
SCR: failure modes documented, costs unknown
This is the thinnest part of our record and we would rather say so than pad it.
We have SCR doser valve and NOx sensor faults documented, and we have what causes them: excessive idling and PTO use. A truck that spent its life sitting and running is a different risk profile from one that spent its life at highway load, and the SCR is one of the places that difference shows up.
We have no SCR part price. No catalyst cost, no doser valve cost, no NOx sensor cost, from any source in the set. If SCR repair pricing is the thing you need, our record cannot give it to you and we are not going to pretend otherwise.
The cause almost every mechanic names
Ask a diesel tech what feeds aftertreatment failure and you get a remarkably consistent answer. One mechanic in our record walks through it plainly: idle the engine and the cooler stops doing its job, so you push raw, dirty air into the intake, and you clog your valves. His prescription is four words long. Do not idle the engines.
That is worth more to you than any price range on this page. Carbon accumulates when the engine loafs. The SCR faults track idling and PTO use. A truck's idle history is a real predictor of what its emissions stack is going to cost you, and it is the one thing the odometer will never tell you.
When the aftertreatment bill starts competing with a rebuild
At some point the question stops being "what does this repair cost" and becomes "is this truck worth repairing." Our parts-price record gives you the other side of that comparison.
From a 2026-08-19 check at dieselrebuildkits.com: a complete Cummins ISX15 engine overhaul rebuild kit is $3,295. The X15 kit is $3,995. The upper gasket set is $795 and the head bolt kit is $395. On the Detroit side, a DD15 in-frame overhaul kit is $4,495, the high-performance variant $4,995, and a fully assembled DD15 cylinder head $3,995.
Those are parts on a shelf, which is the easy half. The hard half is what opening the engine reveals. One documented in-frame job in our video record ran $4,600 in parts and labor for intake valve reseating, valve springs and new cam bearings, and the mechanic who filmed it was explicit that this figure covers those items only, not the whole in-frame. He filmed it because he thinks the in-frame numbers circulating online are exaggerated and he wanted real ones on the record.
At the top end, we have one full dealer-performed overhaul invoice read out on camera on a Volvo D13: $34,090.58. One case, one dealer, one truck. Not a quote for yours. But it is a real ceiling from a real job, which is more than most rebuild-cost articles can say.
So the honest framing of the rebuild question is this. A single documented aftertreatment breakdown ran $9,136.54 all in. A documented full overhaul ran $34,090.58. Those two numbers are the poles you are deciding between, and where a specific truck sits depends on how many aftertreatment events you think you are buying and how much life is left underneath.
What to do about it when you are buying
Pull the fault codes, including derate history, before money moves. A code read is cheap and it tells you whether this truck has been derating on aftertreatment faults or whether somebody cleared the lights and kept driving.
Ask for the DPF ash-cleaning records against that 250,000 to 400,000 mile interval. Ask what the truck did for a living, because idle hours drive EGR carbon and SCR faults and no seller volunteers that. Ask whether the EGR valve has ever been replaced, and if the answer is yes, ask for the invoice, because a documented $1,350 part is worth real money to you as a buyer.
And price the unknowns as unknowns rather than as zero. On a truck with no aftertreatment service history, the honest base case is that the stack needs work, you do not know which piece, and the one complete invoice we have for that scenario came to $9,136.54 with the hotel included.
Engine-generation detail and the full failure record: the Cascadia ISX15, Peterbilt 579 X15 and Mack Anthem MP8 records. Full purchase math: the used ISX15 and X15 buying guide.