A fault code tells you where to look. It does not tell you what to replace. That gap is most of what separates a repair that works from a sequence of parts purchases that eventually stumbles onto the answer.
What follows is symptom-first, because that is how you actually experience a problem from the driver's seat. For each one: the usual cause, the other real possibilities, and whether it is safe to keep rolling.
Jump to a symptom
The truck derated / lost power suddenly
Usually: aftertreatment. Nine times out of ten in Polk County.
A sudden, dramatic power limit with a warning light is the ECM protecting the engine because the emissions system is not doing its job. The most common cause here is a soot load the truck could not burn off, because it runs short distribution routes and never gets sustained highway temperature. DEF quality problems, a failed NOx sensor, a dosing injector that has crystallized shut, or an exhaust leak bleeding off regeneration temperature all produce the same result.
Also possible: a low coolant level condition, an oil pressure fault, or a turbo actuator problem — all of which can trigger protective derates.
Keep driving? Get off the interstate. A truck limited to 5–20 mph in an I-4 travel lane is a hazard to itself and everyone behind it. Reach the nearest exit or the widest shoulder available, then stop. Aftertreatment & DPF services →
A regen that starts but never finishes
Usually: the system cannot reach or hold regeneration temperature.
Common causes, roughly in order: an exhaust leak upstream of the filter letting heat escape, a failed exhaust temperature sensor giving the ECM bad data, a dosing injector not spraying, a stuck EGR valve, or a filter loaded so far past recoverable that no regen will save it.
An important distinction: soot burns off in a regen. Ash does not. Ash accumulates permanently over hundreds of thousands of miles from oil and fuel additives. If your filter is ash-loaded, more highway miles will not help — it needs professional cleaning or replacement. A tech who measures actual soot and ash values can tell you which one you have in a few minutes.
Keep driving? Usually yes, briefly, but do not ignore it. A failed regen is the step immediately before a derate, and derates never happen somewhere convenient.
Nothing happens when I turn the key
Usually, in Florida: the batteries.
Drivers who came up north expect batteries to die in the cold. Down here it is heat that kills them — sustained mid-90s temperatures cook the plates and boil off electrolyte over a summer. The failure mode is what catches people: a heat-degraded battery still cranks a warm engine perfectly well right up until the morning it does not crank at all. No warning, no gradual decline you would notice.
Also possible: a corroded or loose ground, a failed starter solenoid (usually a single loud click), a dead starter, or an ignition switch fault.
What a jump start actually tells you: if it fires and runs fine, the batteries or a connection are the problem. If it fires and dies again shortly after, your alternator is not charging and you have been running on stored power. Which is why a jump alone is often just relocating the same breakdown a few hours down the road. Mobile electrical repair →
It cranks but won't fire
Usually: fuel, or air in the fuel.
A diesel that spins normally but will not catch is almost always a fuel delivery problem. The classics: air in the system after a filter change that was not properly primed, running the tanks dry, a leak on the suction side pulling air in, a failed lift pump, or a filter so restricted it cannot flow enough at cranking speed.
Also possible: a sensor telling the ECM not to inject — cam or crank position, for instance — or a shutdown the truck has decided on for a reason it will happily explain if someone plugs in and asks.
Important: adding fuel to a truck that ran dry is only half the job. The air in the lines has to be purged or it will crank forever without firing. Fuel system repair →
Gradual power loss under load
Usually: a restriction — either air going in or exhaust coming out.
The classic version is a fuel filter slowly plugging: the engine idles and cruises light just fine, then starves the moment you ask for real power. Equally common is a boost leak — a split charge air cooler boot or a loose clamp — or a turbo with a sticking variable geometry mechanism. And in modern trucks, a partially plugged aftertreatment system creates backpressure that chokes the engine without setting a dramatic code yet.
Also possible: a restricted air filter (very common on trucks working the phosphate roads around Mulberry and Bartow, or agricultural properties around Plant City and Kathleen), or injector wear.
Keep driving? Usually, at reduced expectations. But it is worth diagnosing before it becomes a derate.
Black smoke
Usually: too much fuel or not enough air — the ratio is wrong.
Restricted intake, a boost leak, a failing turbo, or an injection problem delivering more fuel than the air charge can burn. On an older truck a puff under hard acceleration is normal. Sustained black smoke is not, and it is also an efficient way to plug a particulate filter, which turns one problem into two.
While you are at it: white smoke usually means unburned fuel or coolant entering the combustion chamber. Blue smoke means oil. Both are worth diagnosing promptly.
Temperature climbing
Usually, in Central Florida: airflow, not coolant.
Here is the diagnostic shortcut worth knowing: if the truck runs fine at highway speed and only overheats in traffic, it is an airflow problem, not a coolant problem. At speed you get ram air through the radiator for free. In stop-and-go the fan has to do all of it. Overheating that appears only in congestion points at the fan clutch, the fan itself, a blocked radiator or charge air cooler, or a damaged shroud.
If it overheats everywhere, look at coolant level, a burst or collapsing hose, the water pump, the thermostat, or a pressure cap that stopped holding.
Keep driving? No. This is the one on this list where the cost of being wrong is enormous. Shut it down. A hose is a few hundred dollars; a warped head, blown gasket or scored liners is five figures and weeks out of service. Do not add cold water to a hot engine either — the thermal shock can crack components. Cooling system repair →
Air won't build, or won't release
Usually: a leak, and it is rarely where you think it is.
Air travels through the system, so the hiss you hear is often nowhere near the actual failure. Common sources: a ruptured line, a chamber diaphragm that has torn, a leaking valve, a glad hand seal, or a governor or compressor that has quit. If the system will not build, the spring brakes stay applied and the truck sits exactly where it is — which is precisely the scenario mobile repair exists for.
The local wrinkle: Florida humidity plus a tired air dryer cartridge puts water in the tanks, and that water corrodes valves from the inside out. Air system problems here often trace back to moisture that has been quietly doing damage for months. Brake & air system repair →
Rough idle or a miss
Usually: one cylinder not contributing properly.
An injector is the usual suspect, and cylinder cutout testing isolates which one quickly without pulling anything apart. Valve adjustment out of spec, air in the fuel, or a genuine mechanical problem in one cylinder are the other real possibilities.
Keep driving? Short distances at reduced load, usually. A persistent miss dumps unburned fuel into the exhaust, which is unkind to the aftertreatment system, so it is not a symptom to live with for a month. Engine diagnostics →
The stop-driving list
Most problems allow you to reach a safe spot. These do not. If any of these are happening, get off the road and shut it down:
- Temperature gauge climbing toward or past the red
- Oil pressure dropping or an oil pressure warning
- Any smoke from the cab, wheel ends or engine bay, or a burning smell
- A wheel end that smells hot or is smoking — a dragging brake can start a fire
- Loss of steering feel or a sudden pull
- Air pressure dropping and not recovering
- Fuel, oil or coolant visibly running onto the ground
- A derate leaving you far below traffic speed on an interstate
Everything on that list is cheaper to deal with stopped than moving. Every single one.