Why does my heavy-duty diesel truck keep going into limp mode with a DEF quality fault?
Your truck enters limp mode with a DEF quality fault because the urea concentration is out of spec — usually from contaminated,…
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Heavy-Duty Vehicles & Fleet
Practical, step-by-step fixes for heavy-duty vehicles & fleet problems — 67 published so far.
Why does my heavy-duty diesel truck keep going into limp mode?
Limp mode activates when your truck's computer detects a serious fault in the emissions, fuel, or transmission systems and limits power to…
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Why does my heavy-duty diesel truck's DEF consumption suddenly increase?
A sudden spike in DEF consumption usually means a leaking DEF injector, a faulty NOx sensor, or a malfunctioning SCR catalyst causing…
See the fixWhy does my heavy-duty truck's battery drain overnight when parked?
A parasitic draw from accessories like telematics, a fridge, or a faulty alternator diode drains the battery overnight. Test with a multimeter…
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Why does my heavy-duty truck's battery keep dying overnight?
A heavy-duty truck battery that dies overnight usually has a parasitic draw from a stuck relay, faulty alternator diode, or an accessory…
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Why is a diesel truck burning more fuel than usual?
A diesel truck burns more fuel than usual when something simple is wrong: under-inflated tires, a clogged air filter, dragging brakes, a…
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Why is my 2018 Freightliner Cascadia getting 2 mpg worse than last month
A sudden 2 mpg drop in a 2018 Freightliner Cascadia usually means a clogged air filter, leaking boost tubes, failing injectors, or…
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Why is my 2020 Freightliner Cascadia getting 1 mpg worse than last month?
A sudden 1 mpg drop in a 2020 Freightliner Cascadia usually means a clogged DPF filter, leaking fuel injectors, low tire pressure,…
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Why is my 2020 Kenworth T680 burning 15% more fuel than spec
A 2020 Kenworth T680 burning 15% over spec usually points to a clogged DPF, faulty injector, low tire pressure, or excessive idling…
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Why is my 2020 Peterbilt 579 suddenly using 15% more diesel fuel?
A sudden 15% fuel economy drop in a 2020 Peterbilt 579 usually stems from a clogged air filter, faulty fuel injectors, low…
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Why is my 2021 Freightliner Cascadia suddenly getting 4.5 mpg instead of 6.5 mpg
A sudden drop from 6.5 to 4.5 mpg usually means a clogged DPF forcing frequent regens, a leaking charge-air cooler hose, bad…
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Why is my 2021 Kenworth T680 suddenly getting 6 mpg instead of 7.5 mpg?
A sudden drop from 7.5 to 6 mpg on a 2021 Kenworth T680 usually means a clogged air filter, low tire pressure,…
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