A warning light came on
Start with urgency, preserve the original evidence, and understand what the code can—and cannot—prove.
Open the right guide ↗
View plans ScanWrench Learning Center
Know what the data
can prove.
A practical, reviewed index for drivers, buyers, technicians, and shops. Learn what the vehicle observed, how signals relate, what requires exact manufacturer access, and where a safe diagnosis must stop.
18in-depth guides
26reviewed DTC references
57defined technical terms
0pay-to-rank conclusions
SEARCH 101 REVIEWED ENTRIES
Search titles, definitions, systems, signals, and diagnostic questions. Every result links to a crawlable permanent page or definition.
START WITH THE QUESTION01 02 03 04
I want to understand my scan
Decode status, freeze frame, readiness, live parameters, and monitor results as one diagnostic record.
Open the right guide ↗I am buying a vehicle
Combine electronic evidence with an independent physical, historical, and mechanical inspection.
Open the right guide ↗I am verifying a repair
Repeat the original conditions and document what passed, what returned, and what remains unknown.
Open the right guide ↗SCANWRENCH ACADEMYInformation is useful.
Information is useful.
Practice changes decisions.
Ten role-based schools combine 40 applied lessons, scored mastery checks, field missions, and 12 interactive diagnostic cases.
THE KNOWLEDGE INDEXFrom first code
From first code
to verified repair.
Each guide is written as a decision aid: observed facts, competing causes, useful evidence, safety limits, and related concepts.
Diagnostic foundations
04 guidesOBD-II diagnostic trouble codes explained
Learn how OBD-II codes are structured, what generic and manufacturer-specific codes mean, and why a code is evidence—not a parts order.
Read guide ↗Stored, pending, and permanent codes
Understand the three OBD-II emissions code channels and how each affects diagnosis, clearing, and repair verification.
Read guide ↗OBD-II readiness monitors explained
Learn what complete, incomplete, supported, and not supported readiness states reveal after a repair or code clear.
Read guide ↗An evidence-first vehicle diagnostic workflow
A repeatable sequence from complaint and preflight through scan, hypothesis, controlled testing, repair, and verification.
Read guide ↗Signals and evidence
04 guidesHow to read OBD-II freeze-frame data
Use engine load, temperature, speed, fuel trim, RPM, and other captured conditions to reconstruct when a fault occurred.
Read guide ↗Building a useful live-data baseline
A practical method for comparing OBD-II live data across cold start, warm idle, steady cruise, and controlled load.
Read guide ↗Short-term and long-term fuel trim guide
Understand STFT, LTFT, bank comparisons, operating-state changes, and the evidence needed before blaming a sensor or fuel component.
Read guide ↗Mode $06 onboard monitor results
Learn what Mode $06 test results can reveal, why identifiers vary, and how to avoid false precision without exact definitions.
Read guide ↗Hardware and access
03 guidesChoosing an OBD-II adapter for iPhone
Compare BLE and Wi-Fi OBD-II adapters, understand iPhone transport limits, and evaluate firmware, protocol, speed, and security.
Read guide ↗Generic OBD-II vs manufacturer-specific diagnostics
A clear boundary between universal emissions data, enhanced module diagnostics, service functions, coding, and programming.
Read guide ↗Secure gateways, OEM access, and diagnostic authorization
Understand why newer vehicles restrict commands, what authenticated access changes, and why key, immobilizer, coding, and programming work need strict authority.
Read guide ↗Battery and electrification
02 guidesHybrid and EV battery diagnostic evidence
Understand cell and block voltage, temperature, current, state estimates, isolation, and why a scan alone may not prove battery health.
Read guide ↗12-volt battery and charging-system diagnostics
Separate open-circuit voltage, cranking behavior, charging strategy, current, ripple, and parasitic draw evidence.
Read guide ↗Buying and repair
04 guidesHow to use an OBD-II scan before buying a used car
A pre-purchase workflow for code status, readiness, freeze frame, VIN consistency, battery evidence, road test, and independent inspection.
Read guide ↗How to verify an automotive part number and fitment
A parts-research workflow that starts with the exact catalog brand and manufacturer part number before comparing sellers and price.
Read guide ↗How to diagnose an intermittent vehicle problem
Use event conditions, repeated scans, voltage and network evidence, controlled logging, and non-destructive testing to catch faults that disappear.
Read guide ↗How to verify a vehicle repair
Prove a repair with before-and-after evidence, comparable operating conditions, monitor completion, and documented residual risk.
Read guide ↗Safety and ownership
01 guidesWhat to do when the check-engine light comes on
A calm safety-first sequence for steady and flashing malfunction indicator lamps, evidence capture, diagnosis, and repair verification.
Read guide ↗Build real mastery.
Ten role-based schools add scored decisions, interactive cases, field missions, and private completion progress to the public library.
Enter the Academy ↗Take the method outside.
Print or save six evidence checklists for the first scan, intermittent events, used-car review, adapter fit, exact parts, and repair verification.
Open checklists ↗Start with the exact code.
Review what the controller observed, possible conditions, the most useful evidence, and the safety screen—without turning a code into a parts order.
Browse 26 code references ↗Translate the scan.
From readiness and fuel trim to gateways, J2534, UDS, state of health, and flash programming.
Browse 57 terms ↗Follow the evidence.
Original case debriefs, access truth, buyer briefs, EV explainers, parts evidence, and fleet workflows in a crawlable public newsletter archive.
Read Dispatch ↗FROM EDUCATION TO EVIDENCELearn the method.
Learn the method.
Capture the record.
ScanWrench keeps codes, readiness, freeze frames, live baselines, and limitations together so the next decision starts with evidence.
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