Diesel Emissions Codes
DPF, DEF, SCR and EGR codes on light and medium duty diesels.
What causes a DPF soot level too high code?
Short answer: Excess soot usually means regenerations are not completing, from short trips, a faulty differential pressure sensor, EGR or injector problems adding soot, or an interrupted regen.
The diesel particulate filter traps soot and burns it off during regeneration. When the computer calculates soot loading above its limit, it sets a code and may reduce power. The question is why the filter is not cleaning itself.
Look at driving patterns first. Trucks that idle a lot or only make short trips may never stay hot long enough to finish a regen. Next, check the differential pressure sensor and its hoses; a cracked hose or plugged port can give false readings. Review regen history in the scan tool if available.
Then find the soot source. A sticking EGR valve, leaking injector, boost leak or turbo problem can produce more soot than the filter can handle. Fix the cause before forcing a regeneration. When you run a service regen, follow the tool's instructions, park outdoors away from anything flammable, and keep people clear of the extremely hot exhaust.
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What does a DEF quality or SCR efficiency code mean on a diesel?
Short answer: It means the computer sees NOx levels after the SCR catalyst that are too high, which can come from poor DEF quality, a dosing problem, a faulty NOx sensor or a worn catalyst.
Selective catalytic reduction systems spray diesel exhaust fluid into the exhaust so the catalyst can convert NOx. NOx sensors before and after the catalyst tell the computer how well it is working. When downstream NOx stays high, it sets an efficiency or quality code and may start a countdown to reduced power.
Test the fluid first. DEF should be about 32.5 percent urea, and a refractometer made for DEF reads it quickly. Contaminated or diluted fluid is common, especially if someone added water or the wrong fluid. Check for crystal buildup at the doser and lines.
Next, run the dosing test with a scan tool to confirm the system delivers the right amount, and check NOx sensor readings for plausibility. Software updates are common for these systems, so check bulletins. Keep DEF off painted surfaces and clean spills promptly.
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What does a glow plug circuit code mean on a diesel?
Short answer: A glow plug circuit code means the control module saw an electrical fault in a glow plug or its circuit, such as an open plug, a shorted plug, a wiring problem or a glow plug module fault.
Glow plugs heat the combustion chamber to help a diesel start, especially when cold. The engine or glow plug control module monitors current through each plug or the circuit, and sets a code when it sees an open, short or out-of-range reading. Codes like P0380 and the cylinder-specific codes in the P0671 range are examples, but exact definitions vary by maker.
Start with service information for the specific code. Check the plug's resistance or current draw against specification, inspect connectors and wiring for heat damage or corrosion, and check the control module's power and ground. A low-amp current probe can compare the current each plug draws.
Glow plugs can seize in the head and break during removal; follow the maker's procedure. Glow plugs and wiring get very hot, so let the engine cool, disconnect the battery as directed, and wear safety glasses.
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What does a NOx sensor code mean on a diesel?
Short answer: A NOx sensor code means the module saw a problem with a nitrogen oxide sensor's circuit, heater, communication or readings, which affects SCR system monitoring and can trigger derate warnings on some vehicles.
Diesel SCR systems use NOx sensors before and after the catalyst to measure how well the system reduces nitrogen oxides. Many NOx sensors include their own control module that communicates with the engine computer over a network.
Codes can point to a sensor heater fault, a communication fault, a reading out of range, or a rationality problem where the sensor disagrees with what the module expects. Exhaust leaks, wiring damage, and DEF problems can all affect readings. Check service information, inspect wiring near hot exhaust, and review sensor data with a capable scan tool before replacing a sensor.
Some vehicles require resets or adaptations after replacement. Exhaust components get extremely hot; let the system cool before working on it and wear gloves and safety glasses.
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What does a DPF differential pressure sensor code mean?
Short answer: It points to a problem with the sensor that measures pressure across the diesel particulate filter, its hoses or wiring, which can cause incorrect soot estimates and regeneration problems.
The DPF pressure sensor compares exhaust pressure before and after the filter to estimate how loaded it is. Codes in the P2452 to P2455 range are examples of DPF pressure sensor circuit codes, though definitions vary by maker. If the sensor or its hoses fail, the module may misjudge soot load, regenerate too often, or not regenerate at all.
Check the sensor hoses first, since they can crack, melt, plug, or be swapped during repairs. Inspect wiring near hot exhaust. Compare sensor readings with the engine off and running against service information.
Fix the sensor fault before forcing a regeneration. During a regeneration, exhaust temperatures get very high: keep the vehicle outdoors or well ventilated, away from flammable materials, and follow the maker's procedure.
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General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.