Faults by symptom

Where to start

• Check Engine light is on — A yellow light means the electronics found a fault and logged a code
• AdBlue (DEF) error — More than half of AdBlue errors come from an empty tank or the wrong fluid
• Power dropped, the truck will not pull — Torque derate is not a fault by itself — it is a protection response: the electronics cut power because they found a problem
• Low oil pressure — This is the most urgent of the common faults
• Engine overheating — Overheating rarely comes alone — a power derate and a protective shutdown usually follow
• Clogged particulate filter (DPF) — The filter clogs when the truck spends a lot of time idling or driving in the city — regeneration never gets a chance to run
• Water in the fuel, clogged filter — In winter, diesel waxing adds to the problem
• Power dropped, errors across the whole truck — If the scanner throws a dozen codes from different systems at once, it is almost never ten separate faults — it is the power supply or the bus
• Engine will not start — The starter cranks but the engine will not catch — that is one thing; the starter does not turn at all — that is another
• Engine stalls while driving — Almost always this is fuel delivery: an air leak, a clogged filter, a failed lift pump
• Engine running rough, misfires — The electronics count misfires per cylinder, so the code names the culprit directly
• Black exhaust smoke — Black smoke means too much fuel or not enough air
• White exhaust smoke — White vapor on a cold start is normal
• Blue smoke, oil in the exhaust — Bluish smoke means oil is entering the combustion chamber
• Fuel consumption has gone up — Consumption rises when the engine is not running in its intended range: not enough air, a leaking injector, a clogged particulate filter
• DPF regeneration will not run — Regeneration will not start if the truck idles a lot or the system cannot raise exhaust temperature
• AdBlue not pumping, dosing error — The pump module is not building pressure or cannot hold it: a blocked line, crystallized DEF, a failed heater
• NOx sensor error — There are usually two NOx sensors — upstream and downstream of the catalyst — and the control unit compares them
• Alternator is not charging — With the engine running, the on-board network should read 27–29 V
• Air pressure will not build — Slow build-up or falling pressure points to a leak, the compressor, or the air dryer
• ABS light is on — The wheel speed sensor is the usual culprit — excess air gap, dirt, a damaged tone ring
• Parking brake will not release — Spring brake chambers release the shoes with air pressure, so the first thing to check is whether the circuit has built full pressure
• Gears will not shift — The automated manual gearbox drops into limp-home mode and leaves you with one or two gears, or neutral
• Clutch is slipping — RPM climbs but speed does not — the discs are not holding torque
• Retarder error — The retarder shuts itself off when the oil overheats or it loses communication with the control unit
• Overheats under load — If the temperature climbs only on grades and drops back on level ground, the problem is usually heat rejection: the fan viscous clutch, a clogged radiator, the intercooler
• Tachograph error — The tachograph broadcasts speed and distance over the bus, so its fault pulls in codes from other systems too — from cruise control to the speed limiter
• Engine brake not working — The engine (compression) brake shuts itself off if the exhaust flap or its actuator is faulty

If you do have a code

A symptom narrows things down, but the code number gives the exact answer. Type the code from the scanner — the reference will tell you plainly whether to keep driving or stop, and how much time you have. Several codes at once — paste the whole list, we will show which one is the root cause.