From grimoire
Diagnoses automotive electrical faults using OBD-II codes, multimeter voltage/continuity tests, and visual inspection. Apply systematic sequence for blown fuses, inoperative accessories, warning lights, or intermittent failures.
How this skill is triggered — by the user, by Claude, or both
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/grimoire:diagnose-electrical-faultThe summary Claude sees in its skill listing — used to decide when to auto-load this skill
Isolate an automotive electrical fault to a specific component or circuit using OBD-II codes, multimeter voltage and continuity tests, and systematic visual inspection — before replacing any parts.
Isolate an automotive electrical fault to a specific component or circuit using OBD-II codes, multimeter voltage and continuity tests, and systematic visual inspection — before replacing any parts.
Adopted by: ASE A6 Electrical/Electronic Systems is a standalone certification reflecting the complexity of modern vehicle electrical systems. Professional diagnostic procedures (per Bosch Automotive Handbook and OEM service manuals) require OBD-II code retrieval and circuit testing before part replacement. AutoZone, O'Reilly, and most parts stores offer free OBD-II scanning — reducing shotgun part replacement. Impact: Modern vehicles have 50–100 Electronic Control Units (ECUs) controlling everything from emissions to comfort features. A "random" symptom (intermittent stall, flickering dash) is rarely random — it has a traceable electrical cause. Replacing parts without testing (the "swapperoo" method) costs $100–$1,000 in unnecessary parts and often doesn't fix the problem. A multimeter and 30 minutes of methodical testing identifies the specific failed component.
OBD-II codes are the fastest path to circuit-level information:
OBD-II code format: P (powertrain), B (body), C (chassis), U (network) + 4 digits.
A code points to a circuit, not always a component. P0300 (random misfire) does not mean "replace coils" — it means test the ignition system circuit. P0128 (coolant temp below threshold) usually means a stuck-open thermostat, not a sensor.
Use a vehicle-specific wiring diagram (Haynes, Chilton, or OEM service manual):
This step is what separates guesswork from systematic diagnosis — you need to know what's in the circuit to test it.
Before touching a multimeter:
Fuse test with a multimeter: a visually intact fuse may have a broken element. Test with continuity mode (beep = good; no beep = blown) or measure voltage on both sides of the fuse — both sides should read battery voltage when the circuit is active.
Voltage testing (circuit active):
Continuity testing (circuit disconnected):
Ground test:
Once circuit is confirmed intact (power good, ground good, wiring good), test the component:
Sensor resistance test: disconnect sensor; measure resistance across terminals; compare to spec (e.g., a coolant temp sensor at 68°F should read ~2,500 Ω for most GM, Ford, and Toyota applications)
Actuator voltage test: apply direct battery voltage to an actuator (injector, solenoid, relay) to verify mechanical function independent of the control module
Control module: if all inputs test good and outputs test good but the system doesn't respond, the ECU itself may be suspect — this is rare but possible; verify with a known-good module before replacement ($200–$600+)
After identifying and fixing the fault:
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