A 2017 Detroit DD15 engine is experiencing persistent low NOx conversion efficiency issues, triggering derate and check engine lights. Numerous components have been replaced and cleaned, yet the problem persists. This article outlines the troubleshooting steps taken so far and seeks further diagnostic suggestions.
Chronic Low NOx Efficiency Despite Extensive Repairs
Initially, the truck presented with a check engine light, derate, and flashing DEF light. A dealer diagnosed a low NOx conversion efficiency code and faulty outlet NOx sensor. They performed module updates and sensor replacement, but the issue remained. Subsequent interventions by the dealer included valve adjustments and air filter replacement, all without success. The dealer ultimately suggested a One Box replacement.
Taking matters into his own hands, the owner retrieved the truck and began independent troubleshooting. Old DEF fluid was replaced, and several sensors (Delta pressure, MAF, air intake temperature, and HC doser), found heavily sooted, were replaced. A DPF regeneration was performed, revealing fluctuating NOx conversion efficiency, initially high (95-100%) before dropping to 0% and then hovering between 20-60%. Doc inlet pressure was 0.4-0.6, with outlet pressure around half that value.
Example of a DD15 Engine
Despite cleaning the DPF filters (two of which were recently replaced) and the One Box, and replacing the inlet NOx sensor, inlet Doc pressure sensor (along with a clogged hose), and soot sensor, the problem persisted even after another regeneration. A new DEF doser and Doc face plug were also installed to no avail.
Suspecting a cooling system issue due to engine temperatures fluctuating between 205-220°F (fan engaging at 220-221°F), the water pump and thermostat were replaced. However, the NOx efficiency issue remained unchanged. DPF temperatures reached 1100°F during regeneration, but NOx efficiency would still exhibit the same erratic behavior, occasionally starting high before plummeting. Engine temperature during regeneration remained in the 205-220°F range.
Example of a DPF Filter
The ACM harness was inspected and found to be undamaged, with ohm readings from the ACM to the outlet NOx sensor within specifications. The problem seems to worsen as the engine temperature rises. Inlet NOx readings are around 400ppm, with outlet readings sometimes exceeding the inlet value. No DEF crystallization was observed.
Potential Causes and Next Steps
The intermittent nature of the high NOx efficiency readings suggests an intermittent fault or a component sensitive to temperature changes. Potential remaining areas to investigate include:
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ACM (Aftertreatment Control Module): Could a malfunctioning ACM be causing erratic control signals?
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Wiring Harness: While visually inspected, could the harness have an intermittent short or open circuit triggered by heat? Thorough testing under operating temperature might reveal a hidden fault.
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Software/Calibration: Could the module updates performed by the dealer have inadvertently introduced a software or calibration issue? Comparing software versions with a known good truck might be beneficial.
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Exhaust Leaks: Any leaks in the exhaust system upstream of the sensors can significantly impact readings and efficiency. A thorough leak check is crucial.
Example of an Exhaust System
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Fuel System: While air/fuel mixture was addressed via valve adjustment, further investigation into fuel injectors, fuel pressure, and fuel quality may be warranted.
The owner is seeking expert opinions and further diagnostic suggestions to resolve this persistent NOx efficiency problem. Have you encountered similar issues, and what other troubleshooting steps could be taken?
