Engine Diagnostics & Troubleshooting
When a heavy-duty engine derates, stalls, loses power or throws fault codes, finding the actual cause matters more than clearing a light. Mobile diagnostics bring scan tools and a mechanic to the truck so the fault can be traced where the truck is parked.
- AvailabilityCalls and messages 24/7
- WhereRoadside, yard or jobsite
- CategoryHeavy-Duty Truck Repair
- On-site repairWhere feasible and safe
About engine diagnostics & troubleshooting.
When a heavy-duty engine derates, stalls, loses power or throws fault codes, finding the actual cause matters more than clearing a light. Mobile diagnostics bring scan tools and a mechanic to the truck so the fault can be traced where the truck is parked.
A mobile mechanic can look at the unit where it stands, whether that is the roadside, a yard or a jobsite. That can avoid a tow and shorten downtime when the fault is suited to on-site work.
Part of Heavy-Duty Truck Repair.
When you might need it.
Any of these is a good reason to call and describe what the unit is doing.
Active fault codes or a derate warning on the dash
Loss of power under load or on hills
Hard starting, rough idle or unexpected shutdowns
Excess smoke, unusual noises or a change in fuel economy
What the service involves.
Reading engine and related system fault codes
Checking sensors, wiring and connections tied to the fault
Visual and functional checks of fuel, air and cooling inputs
Explaining the findings and the repair options
Causes and good habits.
These are possibilities, not a diagnosis. The only way to know is to test the unit.
Possible causes
- Failed or dirty sensors and wiring faults
- Fuel or air supply restrictions
- Emissions system faults that trigger derates
Practical tips
- Note when the fault appears (load, hills, cold start)
- Write down any codes on the dash before they clear
What to expect.
Some faults need shop equipment, major parts or a controlled environment, and in those cases a mobile visit may end with a diagnosis and a recommended plan.
On-site repair depends on the fault, parts, vehicle condition, safety and service feasibility.
(437) 601-8551Before you call
- Exact location, and direction of travel on a highway
- Truck or trailer type, make and year if known
- What happened and what warnings you see
- Whether the unit is somewhere safe
- A number we can call back
From the first call to the fix.
Call or message
Tell us where the unit is.
Describe the fault
Symptoms, lights, noises, leaks.
Agree the plan
Feasibility and dispatch, discussed honestly.
Diagnose and repair
On site where it is possible and safe.
Services that often go together.
Check Engine Light Diagnostics
A warning light is a symptom, not a diagnosis.
Sensor Replacement
Modern trucks rely on sensors to manage the engine, emissions and braking.
DPF / SCR System Diagnostics
Emissions systems cause many derates on modern diesels.
Engine Repair
Engine repair covers the faults found after diagnosis, from leaking components and failed accessories to problems that affect how the engine runs.
Engine Diagnostics & Troubleshooting by area.
Choose your community for local information.
Frequently asked questions.
Can engine faults be diagnosed at the roadside?
Often the first stage, reading codes and checking the obvious causes, can be done on site. Whether the repair itself can be finished there depends on the fault, parts availability and safety.
How do I request this service?
Call (437) 601-8551 or message us on WhatsApp with your location and the issue. For non-urgent work, use the contact form.
Can every repair be done on site?
No. It depends on the fault, parts, vehicle condition, safety and feasibility.
Stranded on the Road? Let's Talk.
Tell us where your truck or trailer is located and what happened. Contact Fellows to discuss mobile repair options and next steps.
(437) 601-8551Have ready: exact location, truck or trailer type, what happened or what warnings you see, whether the unit is somewhere safe, and a callback number. Timing, dispatch and on-site repair depend on location, availability, the fault, parts and safety.