Selling a diesel with DPF, DEF or EGR trouble

So the first question is not "how bad is the fault?" It is "is everything still on the truck?"

Three systems, one exhaust

Modern diesels clean their exhaust in stages, and EPA's own list of emissions hardware names all three that sellers ask about (EPA):

• EGR, exhaust gas recirculation, which sits in the engine.
• DPF, the diesel particulate filter, in the exhaust.
• SCR, selective catalytic reduction, also in the exhaust. SCR is the part that uses diesel exhaust fluid. EPA says that since 2010 nearly all on-road diesel trucks have used DEF in SCR systems to cut nitrogen oxides (EPA).

Each one can fail on its own.

Why a DEF fault can strand the truck

A DEF fault can take a diesel off the road. EPA says DEF systems "can force a vehicle to drastically reduce speed or become inoperable" when the fluid runs out or a system sensor fails (EPA).

One manufacturer bulletin shows how the steps play out. For a single Detroit Diesel engine family, filed with NHTSA (service bulletin):

1. An amber low-DEF icon lights when the tank drops below one-eighth full, and more dash messages follow.
2. When the tank reads empty, speed is capped at 55 mph and the engine is derated.
3. Ignore the empty warning and the cap drops to 5 mph.

The same bulletin says tampering with aftertreatment parts, or filling with DEF of the wrong quality, leads through lamp warnings and power cuts to the same 5 mph limit. Other engines have their own schedules, so read your owner's manual for yours.

EPA has said it is issuing guidance to manufacturers on changing DEF systems on new and existing diesels to reduce derates that can drop speed as low as five miles per hour. Its page does not say which trucks will get changes or when, so don't count on one arriving before you sell.

A failed part versus a delete

Federal law draws the line at what a person does to the hardware. The Clean Air Act makes it unlawful to knowingly "remove or render inoperative" an emissions device after the vehicle has been sold to its first owner (42 U.S.C. 7522).

EPA describes two ways that happens (EPA):

• Pulling hardware. Taking off the EGR, the DPF or the SCR system.
• Tuning. Replacing or changing the engine software or calibrations, including tunes that keep the onboard diagnostics from noticing the changes.

The law also bars making, selling, offering for sale or installing aftermarket parts that defeat emissions controls. EPA's 2020 enforcement policy names diesel particulate filters, diesel oxidation catalysts and SCR systems among the equipment covered (EPA policy).

What that can cost, under that 2020 policy: up to $4,819 per act of tampering for an individual and up to $48,192 for manufacturers and dealers, plus $4,819 for each defeat device. EPA notes the amounts rise with inflation, so today's figures are higher than those. The policy also says many states ban tampering with in-use vehicles, and some ban dealers from selling tampered ones.

A clogged filter that is still bolted on is a worn part. A straight pipe where the filter used to be is a different conversation with any buyer.

What a careful buyer will check

California's air board tells used-diesel shoppers to confirm the factory or retrofit particulate filter is present and working before they buy. Its warning is direct: if the filter core is cracked or missing, it "can cost thousands of dollars" and violate the state's anti-tampering law, Vehicle Code 27156 (CARB).

Expect the same questions from anyone who knows diesels:

• Is the DPF there, and is it the original or a legal replacement?
• Has the engine been tuned?
• What does the dash show right now?
• Has the truck been in a derate, and how far did the speed drop?

If the truck has been tuned or deleted, EPA warns that tampering can also void the vehicle warranty (EPA). That changes what a buyer will pay for it.

Is any of this under warranty?

Check before you spend on a repair or sell. EPA's warranty guidance describes three specified major emission control components with coverage lasting 8 years or 80,000 miles (the earlier of the two) on 1995 and newer vehicles (EPA Q&A). Catalytic converters are the first one named. We could not confirm whether a DPF or DEF part counts as one of the other two, so ask a dealer to run your VIN and tell you in writing. Our page on maker warranty extensions covers what else to look for.

California and Colorado at the sale

These two states have rules that bite at the moment of sale.

California. A Smog Check certificate is needed at change of ownership in every program area, except gas-powered cars no more than four model years old (BAR reference guide). Diesels of model year 1997 and older are exempt at sale, and so are diesels over 14,000 lb GVWR (BAR). The smog rule also drops away when the vehicle changes hands to be wrecked or dismantled (California DMV).

Colorado. Inside the state's testing zone, which covers metro Denver and the North Front Range, diesels skip testing for their first four model years. Check the program rules before a sale there (Colorado DMV).

Emissions testing by state, for a diesel seller

The table shows whether each state runs an emissions test, from verified state records. It does not say whether a deleted diesel can be sold privately there, which we did not confirm outside California.

One row matters more for diesel owners than it looks. Vermont fails a vehicle whose OBD connector is removed or not working (Vermont DMV).

Repair it or let it go

Fixing makes sense when:

• a dealer confirms in writing that the failed part is under warranty
• the fault is narrow, such as low fluid or one sensor, and the rest of the truck is sound
• you need the truck and the repair quote is well under what it is worth running

Selling as it sits makes more sense when:

• the truck is deleted or tuned and you don't want to pay to put the hardware and software back
• a missing or cracked filter core puts you in the "thousands of dollars" range CARB warns about
• it is stuck at 5 mph and the engine has other problems too
• the repair quote is close to the truck's value; our guide on repairs that cost more than the car walks through that math

If it no longer moves under its own power, our junk truck page covers the rest of the sale.

Selling the truck to us

We buy whole non-running and junk cars, and you can sell the whole car as it sits, with free pickup anywhere in the US. For an offer, send us:

• year, make, model and mileage
• the warning lights showing, and any derate the truck is in
• whether the DPF, SCR and EGR are all still on it, and whether the engine has been tuned

Being exact about that last point keeps the deal clean on both sides. Want to talk it through first? Call (877) 421-6008.