Aftertreatment
The engine is usually fine
What strands people on a modern truck is the exhaust hardware bolted on to meet each emissions tightening, and the derate the regulations require when that hardware reports a problem. The bottom end is rarely the thing that ends your week.
Derate is the part nobody explains before you buy. When the system decides it is out of compliance it does not simply warn you: it progressively limits engine power, and at the end of the escalation it limits you to a walking pace. That behaviour is mandated rather than a manufacturer being punitive, and it is why an aftertreatment fault is a towing event rather than a next-week event.
4
of 13 mean stop driving
Not because the truck will break, but because the derate will strand it somewhere worse.
$1,150
median repair
Typical mid-range cost across every fault here. Budget it as a running cost, not an event.
$6,000
worst case
SCR conversion efficiency below threshold, at the top of its range. Diagnose properly before replacing that part.
Diesel particulate filter
3 faults
The filter catches soot and periodically burns it off, which is a regeneration. Regens cost fuel and time, and a filter that will not regen is a truck that will not move. Everything in this group is downstream of one question: is the truck getting hot enough for long enough?
Regeneration will not complete
What you see
The regen light stays on or keeps returning, a parked regen aborts partway, and soot load climbs rather than resetting.
What it is
Usually the truck is not getting hot enough for long enough: short runs, long idling, extended light-load work. Sometimes a failed doser, a leaking exhaust joint upstream, or a filter that is genuinely full of ash rather than soot.
Does it derate
Escalating. A warning first, then power limiting, then in the final stage a speed limit low enough that you are not going anywhere.
Making it less likely
Run it hot. If the work is short-haul and low-load, plan a highway run every few days, and treat a regen request as something to act on that shift rather than tomorrow.
Filter full of ash
What you see
Regens complete but come round far more often than they used to. Back pressure high. Often around 300,000 to 400,000 miles.
What it is
Regeneration burns off soot, not ash. Ash comes from engine oil additives and it accumulates permanently until the filter is physically cleaned or replaced.
Does it derate
Not usually, until back pressure crosses a threshold. The symptom is fuel and time before it is a warning light.
Making it less likely
This one is scheduled maintenance rather than a failure. Cleaning is a few hundred dollars; a new filter is thousands, so budget the clean at the interval rather than waiting.
Doser injector failure
What you see
Regens fail to raise exhaust temperature. Sometimes raw fuel smell, sometimes a doser fault code directly.
What it is
The doser sprays fuel into the exhaust to create the heat a regen needs. It carbons up and blocks, or it leaks.
Does it derate
Follows from failed regens rather than directly.
Making it less likely
Little you can do beyond keeping the whole system healthy so regens are less frequent.
Diesel exhaust fluid and SCR
4 faults
A second fluid dosed into the exhaust to convert nitrogen oxides. It is 32.5% urea and 67.5% deionised water, it freezes at about minus 11 Celsius, and it has a shelf life. Most DEF faults are quality, crystallisation or a dosing unit rather than the catalyst itself.
DEF quality fault
What you see
A DEF quality code and a countdown to derate, often within hours of a fill.
What it is
Contaminated, diluted, old or frozen-and-thawed fluid, or water put in the DEF tank. The quality sensor is reading real chemistry, and it is usually right.
Does it derate
Yes, and quickly. Emissions regulations require it, and the escalation to a crawl is measured in engine hours rather than weeks.
Making it less likely
Buy DEF from somewhere with turnover, do not store it above about 25 Celsius, and never top a tank from an open container that has been sitting in a side box all summer.
DEF crystallisation
What you see
White crusty deposits around the dosing unit or in the decomposition tube, poor conversion efficiency codes.
What it is
DEF that is dosed but not fully vaporised leaves urea deposits behind. Extended idling and low exhaust temperatures make it far worse.
Does it derate
Through the conversion efficiency fault it causes, so yes eventually.
Making it less likely
The same as everything else here: heat and load. Idling is the enemy of every part of an aftertreatment system.
DEF pump or header failure
What you see
DEF level reads wrong or not at all, pressure faults, no dosing.
What it is
The pump and the tank header unit, which carries the level and quality sensors, are among the most commonly replaced parts on a modern truck.
Does it derate
Yes. The system cannot dose, so it cannot comply.
Making it less likely
Keep the tank reasonably full in winter so there is less air and less freeze-thaw cycling, and do not let it sit empty.
SCR conversion efficiency below threshold
What you see
A conversion efficiency code with no obvious symptom. The truck drives normally right up until the derate.
What it is
The catalyst is not converting enough nitrogen oxide. Could be the catalyst, could be dosing, could be an upstream NOx sensor reporting badly, could be a small exhaust leak.
Does it derate
Yes, on a countdown.
Making it less likely
Diagnose properly before replacing the catalyst. This code is frequently a sensor or a leak, and the catalyst is the most expensive thing it could be.
Exhaust gas recirculation
3 faults
Some exhaust is fed back into the intake to cut nitrogen oxides, which means routing hot, sooty gas through a cooler and a valve. Both foul. This is the oldest of the emissions systems and still the most common source of expensive failures.
EGR cooler failure
What you see
Coolant loss with no external leak, white smoke, pressurised coolant tank, sometimes overheating.
What it is
The cooler passes engine coolant around hot exhaust gas. When it cracks internally, coolant goes into the exhaust stream or the intake.
Does it derate
Not directly, but a truck losing coolant is a truck about to have a much larger problem.
Making it less likely
Coolant condition. The additive package protects these, and neglected coolant kills EGR coolers early. Test it rather than looking at it.
EGR valve sticking
What you see
Power loss, black smoke, poor fuel economy, position codes.
What it is
Soot builds up on the valve until it no longer moves freely through its range.
Does it derate
Usually a warning and reduced power rather than a full derate.
Making it less likely
Heat and load again. Short-haul, heavily idled trucks foul EGR valves several times faster than highway trucks.
Variable geometry turbo actuator
What you see
Low power, poor boost, slow throttle response, actuator position codes.
What it is
The vanes that vary the turbo's geometry are in the exhaust stream and they soot up, so the actuator can no longer move them where it is asking.
Does it derate
Reduced power, and it will not pass an inspection cleanly.
Making it less likely
Same story. Sustained load keeps the vanes moving and keeps them clean.
Sensors and wiring
3 faults
Every part of the above is monitored, and a sensor that lies is indistinguishable from the fault it reports until somebody tests it. A meaningful share of aftertreatment repair bills are sensors and connectors rather than the expensive part they pointed at.
NOx sensor failure
What you see
SCR efficiency or NOx codes, often with no drivability symptom at all.
What it is
There are usually two, upstream and downstream, and they live in hot, dirty exhaust. They are consumables with a life, not permanent fittings.
Does it derate
Yes, through the efficiency fault they trigger.
Making it less likely
Nothing much. Treat them as wear items and be relieved when the expensive-sounding SCR code turns out to be one.
Delta pressure sensor and lines
What you see
Soot load readings that make no sense, regens that trigger constantly or never.
What it is
The sensor measures pressure either side of the filter through two small tubes. The tubes crack, block with soot, or get their fittings crossed after a repair.
Does it derate
Through wrong regen behaviour rather than directly.
Making it less likely
Have the lines checked whenever the filter is off, because that is when they get damaged.
Wiring and connector corrosion
What you see
Intermittent codes that clear and return, several unrelated faults at once, faults that change with road vibration or weather.
What it is
Corroded pins, chafed harnesses and water in connectors. Common on a truck that has run winters with salt.
Does it derate
Depends entirely on what the confused sensor is reporting.
Making it less likely
The single most useful diagnostic habit here: several unrelated aftertreatment codes appearing together is far more likely to be one harness than four failed parts.
The one habit that prevents most of this
Heat and load. Nearly every fault on this page is made more likely by short runs, light loads and long idling, because the system needs sustained exhaust temperature to keep itself clean. A truck that idles all night and runs 200 miles a day will foul an EGR valve several times faster than one that runs highway.
Several codes at once is usually one harness
A meaningful share of aftertreatment bills are sensors and connectors rather than the expensive part they pointed at. If four unrelated systems fault together, the cheap explanation is one corroded connector, and it is worth ruling out before anybody quotes you for a catalyst.
What this costs per mile
Put a realistic aftertreatment allowance into the maintenance line of the cost-per-mile calculator rather than hoping. On a post-2010 truck this is not an exception to your maintenance budget; it is a large part of what the budget is for.
Costs are typical independent-shop ranges in the US and move a long way with location, dealer against independent, and whether a part is remanufactured. They are here to show the order of magnitude, which is what you need when deciding what to budget.