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Use when a check engine light is illuminated and you need to identify the fault code, assess severity, and decide next steps
Install with Codex or Claude Copy this prompt, paste it into Codex, Claude, or another assistant, and let it review the skill page and install it for you.
Based on SOC occupation classification
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Use when an organization is forming any standing committee — a risk committee, a compensation committee, an ethics committee, or any other recurring decision-making body — writing a formal charter that defines the committee's scope, authority, and reporting line before it begins operating, rather than letting the committee's mandate remain implicit and subject to later dispute.
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Use when an organization repeatedly experiences slow or contested strategic and operational decisions because it's unclear who has actual authority to decide — explicitly mapping decision types to the specific role or body with final decision authority, distinct from who merely provides input, rather than leaving decision authority ambiguous and re-litigated each time a similar decision arises.
| name | diagnose-check-engine-light |
| description | Use when a check engine light is illuminated and you need to identify the fault code, assess severity, and decide next steps |
| source | SAE J1979 OBD-II diagnostic standard; NHTSA vehicle emissions standards; ASE Automotive Service Excellence diagnostic certification curriculum (A6/A8) |
| tags | ["automotive","troubleshooting","obd2","check-engine"] |
| verified | true |
Systematically identify the cause of an illuminated check engine light using OBD-II codes and symptom correlation to determine urgency and repair path.
Adopted by: All US vehicles 1996+ (mandated by EPA OBD-II regulations), ASE-certified technicians, NHTSA emissions compliance programs, AAA diagnostic service network
Impact: OBD-II standardization (SAE J1979) enables diagnosis in minutes vs. hours of manual testing; correct code interpretation prevents $200–$1,500 in unnecessary part replacements; misdiagnosed CEL causes 35% of failed emissions tests (EPA data)
Why best: The SAE J1979 standard ensures any compliant scanner reads all 10,000+ standardized DTCs from any manufacturer; systematic correlation of code + symptoms + freeze-frame data eliminates guessing
Sources: SAE J1979 "E/E Diagnostic Test Modes" standard; NHTSA OBD-II enforcement; ASE "Automobile Test A6 (Electrical/Electronic Systems)" and "A8 (Engine Performance)" certification curricula
Assess light behavior — Steady light: non-critical, engine likely still driveable; flashing/blinking light: active misfire causing catalytic converter damage — stop driving, tow to shop.
Note symptoms — Before scanning, record: any rough idle, power loss, unusual smells, noises, fuel smell, smoke color; symptoms narrow diagnosis before the code is read.
Connect OBD-II scanner — Plug scanner into DLC (Data Link Connector) under the dash, driver's side; turn key to ON (not start) or start engine per scanner instructions; for Bluetooth scanners, pair to app first.
Read all stored DTCs — Retrieve all pending, current, and historical codes; a pending code (P0xxx with "pending" flag) has triggered once but not yet confirmed; record every code number.
Decode each DTC — First digit: P=powertrain, B=body, C=chassis, U=network. Second digit: 0=generic SAE, 1=manufacturer-specific. Third digit: system (1=fuel/air, 2=injector, 3=ignition, 4=emission, 5=speed/idle, 6=output, 7/8=transmission). Decode using scanner database or NHTSA lookup.
Read freeze-frame data — Freeze frame captures engine parameters at the moment the fault set: RPM, coolant temp, fuel trim, load, MAP/MAF reading; this context identifies conditions that triggered the fault.
Prioritize by severity — Critical (stop driving): P0335 (crankshaft sensor), P0017 (cam/crank correlation), P0300-series misfires (flashing CEL). Moderate (repair soon, 1–7 days): P0420 (cat efficiency), P0171/P0174 (lean condition), P0440-series (EVAP). Minor (schedule at next service): P0128 (thermostat), P0446 (EVAP vent).
Verify with live data — Run live data stream for the suspect system: for O2 sensor codes, watch upstream/downstream O2 switching patterns; for fuel trim codes, watch STFT and LTFT at idle and cruise; deviations confirm the fault vs. sensor noise.
Perform basic visual inspection — Check: fuel cap seated and sealed (30% of EVAP codes are loose caps), obvious vacuum hose cracks, oil cap off, air filter housing open, coolant level, obvious wiring damage near heat sources.
Decide repair path — DIY-appropriate: gas cap, air filter, O2 sensor (most vehicles), mass airflow sensor, PCV valve. Shop-required: catalytic converter, transmission codes, internal engine, ABS/SRS (safety systems). Clear codes only after repair; re-drive 50–100 miles to confirm readiness monitors complete.
