Diagnostic guides
Practical guides for scanners, P/B/C/U letters, lean vs rich air–fuel mixture, severity badges, J1939 SPN/FMI, freeze-frame, and a glossary of common acronyms.
How to read an OBD-II scanner: a beginner’s guide
Learn what an OBD-II scanner shows, how to connect it safely, and how to turn codes into a sensible diagnosis—without clearing faults too early.
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P/B/C/U codes explained: what each letter means
Decode the first letter of an OBD-II trouble code—powertrain, body, chassis, or network—and use it as a smarter starting point before you buy parts.
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Freeze frame data: reading the snapshot behind a code
Freeze frame records what the engine was doing the moment a code set. Learn which values matter, how to turn them into a diagnostic path, and why clearing codes too early throws the evidence away.
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Active, pending, and permanent codes: what the status means
A stored code is not one thing. Learn the difference between pending, confirmed, and permanent DTCs, when the check-engine light actually turns on, and why status matters for inspections and intermittent faults.
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Multiple codes at once: how to triage instead of guess
Six codes on the screen does not mean six repairs. Learn how to sort network, power, and cascade faults from root causes, and in what order to attack them.
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Severity levels: Stop, Check soon, OK to drive
What the three severity badges on each fault-code sheet mean, how they relate to commercial-vehicle roadside criteria (CVSA), and what they do not replace.
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SPN/FMI for heavy trucks: understanding the J1939 system
Learn how heavy-duty diagnostic trouble codes use Suspect Parameter Numbers and Failure Mode Identifiers on the SAE J1939 network—and how that differs from passenger-car OBD-II letters.
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Glossary of diagnostic acronyms
Short definitions of ABS, DPF, SCR, DEF, SPN, FMI, and other acronyms used on TruckCodeLookup code sheets.
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P0171: what does “system too lean” mean?
“Lean” is an air–fuel ratio problem—not oil pressure, not general airflow, and not low power by itself. Stoichiometry (~14.7:1), fuel trims, and the three cause families behind P0171.
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