Built in the shop
Diesel technician and truck driver tools
Fault-code meaning, a test order you can follow, CDL written-test practice, and the numbers an owner-operator has to know. Written by a diesel technician with 30+ years of engine and electrical work behind him — not by a marketing team. Fault-code pages and CDL tests are free. The guided diagnostics and driver tools are $14.99 a month.
If you turn wrenches
For technicians in a shop, a fleet bay, or a service truck on the side of the road.
Start from the code, not a guess
Enter the SPN and FMI, or the engine-specific code, and read what the ECM is actually reporting — circuit, expected range, and what a high, low, open or erratic reading points to.
Test in order
Each fault comes with a sequence: what to check first, what reading confirms it, and what to move to when the check passes. It keeps you off the parts cannon.
Correlate multiple codes
Codes rarely show up alone. The diagnostic takes the whole set and tells you which one is the cause and which ones are downstream of it.
Document the job
Save the truck by VIN or serial, attach photos of the connector or reading you found, and print a work order when the job closes.
If you drive the truck
For company drivers, owner-operators, and anybody still working on the CDL.
Know what the light means
Before you call anybody: whether the code is a derate risk, an aftertreatment problem, or something you can finish the run with.
Talk to the shop straight
Walk in with the code, the symptom and the tests already run. It shortens the conversation and the bill.
Get your CDL, free
Eight chapters of written-test practice — general knowledge, air brakes, combination vehicles, pre-trip, HazMat, tanker, doubles/triples, passenger — with explanations on every answer.
Run your own numbers
Cost per mile, break-even rate, load analyzer, fuel and DEF, IFTA. The numbers that decide whether a load is worth hooking to.
Where to start
Fault code reference
Free J1939 SPN and FMI pages plus engine-specific codes. Meaning, likely causes, and the tests that confirm them.
Guided diagnostics
Your VIN or serial, your codes, your symptoms — turned into a test order with saved per-truck history.
Free CDL practice tests
All eight chapters, no card. Email only, so your progress is there next time.
Owner-operator calculators
Cost per mile, break-even, startup cost and profitability, with real inputs and real results.
Training centers
Cummins ISX, Detroit DD13/DD15 and Aftertreatment Mastermind — modules, quizzes and case simulators.
Who writes this
Christopher Scott Sneed — engine builder and diesel tech, 30+ years on Cat, Cummins, Detroit, Mack, Volvo and International.
What it will not do
It does not hand you a guaranteed diagnosis, and it does not replace a qualified technician or OEM service literature. Verify every procedure, torque spec, pinout and expected reading against OEM service information before you work on the truck. Safety-critical work belongs with a qualified tech. Blue Collar Empire is not a DMV or testing authority — passing a practice test does not issue a license or endorsement.
Common questions
- What does this page cover?
- It is the starting point for two groups: diesel technicians who need fault-code meaning and a test order, and truck drivers who need to know what a code means for the load they are under. Both sides run on the same Truck Commander membership.
- Do I have to pay to look up a fault code?
- No. The SPN, FMI and engine-specific fault-code pages are free to read. You pay when you want the guided diagnostic that takes your VIN or serial, your codes and your symptoms and builds a test order you can save per truck.
- Are the CDL practice tests free?
- Yes. All eight written-test chapters are free. You enter an email address to save progress — no card and no Pro membership.
- Which engines and systems are covered?
- Cummins ISX, Detroit DD13 and DD15, PACCAR, CAT, International, Mack and Volvo, plus universal J1939 SPN/FMI faults. Aftertreatment work — DPF, DOC, SCR and DEF — is covered in depth because that is where most shop hours go.
- Will it tell me exactly what part to replace?
- No, and you should not trust anything that claims it will. It gives you the order to test in and the reading that confirms or clears each step. Verify every procedure, spec, pinout and reading against OEM service information before you work on the truck.
- What does it cost?
- $14.99 a month or $99.99 a year. No free trial. Cancel anytime from your account.
One membership, both sides of the truck
$14.99 a month or $99.99 a year. No free trial. Cancel anytime.