Explainer
Diesel Truck Terms in Plain English: DEF, DPF, EGR, VGT, FICM, CP4 and 23 More
Published · 8 min read
A diesel repair quote can read like alphabet soup: "replace NOx sensor, force regen, check VGT actuator, clear DTCs." This page translates. Each term gets a plain-English meaning, the trucks you will meet it on, and how often owners mention it in their complaints to NHTSA, which is a rough guide to how much it will matter to you.
If you only learn three, learn DEF, DPF and regeneration. Every diesel pickup sold since about 2013 has all three, and they explain most of the warning lights a new owner sees.
Emissions: the parts added since 2007
- DEF diesel exhaust fluid, AdBlue
- A urea and water solution in its own tank (blue cap). The truck sprays it into the exhaust, where it turns nitrogen oxides into nitrogen and water. You refill it like washer fluid. More
- SCR selective catalytic reduction
- The catalyst where DEF does its work. When owners say "the DEF system", they usually mean the SCR system: tank, heater, pump, injector, catalyst and sensors. More
- DPF diesel particulate filter
- A filter in the exhaust that traps soot so it does not leave the tailpipe. It fills up and has to be cleaned by burning the soot off. More
- Regeneration regen
- The truck heats the exhaust to burn the soot out of the DPF, sometimes while you drive. Short trips that keep interrupting it are a common reason a DPF clogs. More
- EGR and the EGR cooler exhaust gas recirculation
- A valve sends some exhaust back into the intake to cool combustion and cut nitrogen oxides. The EGR cooler, a small radiator cooled by engine coolant, cools that exhaust first. A cracked cooler leaks coolant into the engine. More
- NOx sensor
- Measures nitrogen oxides before and after the SCR catalyst, so the computer can dose DEF and check that it works. A failed sensor can set a DEF warning even when the fluid is fine. More
- Derate limp mode, reduced power
- The engine computer limits power or speed to protect the engine or to enforce an emissions fault. The best-known case is the DEF countdown that ends at 5 mph. More
Air: turbos
- Turbo turbocharger
- An exhaust-driven fan that pushes more air into the engine. Every diesel here has at least one; the 6.4 Power Stroke has two. More
- VGT variable geometry turbo
- A turbo with movable vanes that change how it responds at low and high speed. Soot can make the vanes stick, and the electric actuator that moves them can fail. More
Fuel: pumps and injectors
- Common rail
- A high-pressure pump fills one shared pipe (the rail) and the computer opens each injector electrically. It is how almost every diesel since the early 2000s injects fuel. More
- High-pressure fuel pump injection pump, HPFP
- The pump that makes injection pressure. The names you will see are the Bosch CP3, the Bosch CP4.2, the Denso HP4, the Siemens K16 (6.4 Power Stroke) and the older VP44 and P7100 on the 5.9 Cummins. More
- CP4 CP4.2
- A Bosch high-pressure pump that can shed metal into the fuel system when it fails, which can turn a pump job into a full fuel-system replacement. More
- VP44
- An electronic Bosch rotary injection pump, cooled by the fuel it receives. A weak lift pump is the usual suspect when one fails. More
- Lift pump transfer pump
- The low-pressure pump that moves fuel from the tank to the high-pressure pump. Cheap, and the first thing to test. More
- Injectors
- The valves that spray fuel into each cylinder. Worn or sticking injectors cause rough running, smoke and hard starts. More
- HEUI and HPOP high-pressure oil pump
- On the 7.3 and 6.0 Power Stroke, engine oil pumped to very high pressure by the HPOP is what fires the injectors. That is why oil leaks and oil condition affect how these engines start. More
- ICP and IPR injection control pressure sensor, injection pressure regulator
- The sensor that reads that oil pressure (ICP) and the valve that sets it (IPR). Scan-tool readings of both are the first checks on a hard-starting 6.0 or 7.3. More
- FICM fuel injection control module
- The 6.0's injector driver: it steps battery voltage up to about 48 V to fire the injectors. Below about 45 V, expect hard starts and misfires. More
Cold starts
- Glow plugs
- Small heaters in each cylinder that help a cold diesel start. Ford, GM and the half-ton diesels use them. More
- Grid heater intake air heater
- Cummins heats the incoming air with an electric grid instead. On the 6.7 the grid's mounting bolt can loosen and fall into the engine. More
Cooling and the engine itself
- Oil cooler
- A heat exchanger where coolant cools the engine oil. On the 6.0 its coolant passages clog, which then starves the EGR cooler. More
- EOT and ECT engine oil temperature, engine coolant temperature
- Two scan-tool readings. On a 6.0 at highway load, a gap of more than about 15 °F between them points to a clogged oil cooler. More
- Head gasket
- The seal between the cylinder head and the block. When it fails, coolant and combustion gas mix, and the repair is a big one. More
- Head studs
- Replacements for the factory head bolts that clamp the heads harder. Whether a stock truck needs them is disputed. More
- Degas bottle coolant reservoir
- The pressurized coolant tank. A cracked bottle or weak cap can mimic a much bigger cooling failure, so check it first. More
Paperwork: recalls, bulletins and codes
- Recall safety recall, emissions recall
- A defect the manufacturer must fix for free. Check any truck by its VIN; an open recall is a free repair. More
- TSB technical service bulletin
- Repair instructions a manufacturer sends to dealers for a known problem. A TSB is not a free repair by itself, but it tells you the problem is known and how to fix it. More
- Extended coverage special coverage, warranty extension
- A manufacturer program that pays for one part longer than the normal warranty, for certain years and mileage. More
- Trouble code DTC, P-code
- The code the engine computer stores when it sees a fault, such as P0087 (low fuel rail pressure). A code points to a system, not always to the broken part. More
Mentions: NHTSA owner complaints on the ten diesel engine families of our engine articles (4,433 unique complaints, pulled 2026-09-07 to 2026-10-16), word-boundary matches of each term and its common spellings. A mention is not a failure.
The three things a new diesel owner should do
- Keep the DEF topped up and buy it sealed and fresh. Running out ends in a speed limit, not a breakdown, but it is an avoidable one.
- Let regenerations finish. If the truck mostly does short trips, a longer highway drive now and then helps the DPF clean itself.
- Run the VIN for recalls and coverage. Our VIN lookup lists the recalls and extended coverage programs that apply to the truck.
What this page does not do
- Rank the terms by failure rate. A mention count shows what owners talk about, including parts that work as designed but confuse them.
- Cover every engine detail. Each engine's platform page and failure maps go further, with sources for each claim.
FAQ
What is the difference between DEF and DPF?
DEF is a fluid you refill, sprayed into the exhaust to cut nitrogen oxides. The DPF is a filter that traps soot and cleans itself by regeneration. A modern diesel has both, and they fail in different ways.
What does regen mean on a diesel?
Regeneration: the truck heats its exhaust to burn the soot out of the particulate filter. It happens on its own, sometimes while you drive. Interrupting it again and again is a common reason filters clog.
Which diesel trucks do not have DEF?
Pickups built before DEF arrived: Ford before 2011, GM before 2011 (the Duramax LML was the first with DEF) and Ram 6.7 pickups before 2013. The 7.3 and 6.0 Power Stroke, the 5.9 Cummins and the early Duramax have neither DEF nor a DPF.
What is a FICM on a 6.0 Powerstroke?
The fuel injection control module: it raises battery voltage to about 48 V to fire the injectors. When its output drops below about 45 V, the 6.0 starts hard and misfires.
Go deeper
- Every engine and its years on one page
- DEF warning and the 5 mph derate
- Engine symptoms, engine by engine · Trouble codes
- Recalls by engine · How we verify and tag facts
Sources. Definitions from manufacturer service bulletins and the engine records on our platform pages, which cite their sources; the FICM voltage figures are shop and owner sources, as in our 6.0 buyer's guide. Mention counts from NHTSA owner complaints (US government public domain), counted with word-boundary patterns. Torque Index takes no paid placement.