1) Purpose and symptoms (why start here)
- Theory: the ignition coil is a transformer that converts 12 V primary current into a high-voltage secondary pulse that makes the spark jump the plug gap. If the coil’s primary or secondary windings, insulation, or driver fail you get weak/no spark → misfire, rough idle, loss of power, poor economy, increased HC/CO and possible catalytic damage.
- Do this first to focus your work and avoid replacing parts unnecessarily.

2) Read codes and capture symptoms (order step)
- What to do: read OBD-II for P030x misfire codes, and look at freeze-frame/live data (RPM, load, misfire counters).
- Why: ECU codes and misfire counts narrow the failing cylinder(s). A persistent cylinder-specific P030x implicates that cylinder’s coil/plugs/wiring first.

3) Visual inspection
- What: with engine cool, remove engine cover. Inspect coil boots, connectors and wiring for corrosion, melting, oil/fuel contamination, and carbon tracks on boots or plug tips. Inspect spark plugs for electrode wear or fouling.
- Why: many coil faults are visible (cracked boots, oil ingress, carbon tracking). A bad plug or fouled boot can mimic coil failure; replacing coil without checking plug/wire wastes time.

4) Simple functional checks (swap test)
- What: swap the suspect cylinder’s coil with a known-good cylinder’s coil and rerun the fault (or watch misfire live data).
- Why/theory: if the misfire moves to the other cylinder, the coil is the cause. If it stays, the problem is plug/wiring/ECU/driver/sensor. This isolates coil vs. other systems without tools.

5) Basic electrical checks
- What: with coils unplugged, measure:
- Primary resistance (between the coil’s primary terminals) with a multimeter.
- Secondary resistance (between high-voltage output and primary terminal or specified points).
- Check power supply: with ignition ON, verify +12 V at the coil power feed and good ground continuity.
- Typical guidance: many Nissan coil-on-plug primary ~0.4–2 Ω and secondary ~8k–15k Ω — consult the service manual for exact D22 values.
- Why/theory: an open or short in primary/secondary tells you the coil windings or internal insulation failed. No supply voltage or bad ground shows wiring/ECU/power feed problem, not the coil itself.

6) Advanced waveform test (if available)
- What: use an oscilloscope or a dedicated ignition analyzer to view coil primary waveform while cranking/running.
- Why/theory: waveform shows driver behavior, dwell time, primary current ramp and collapse. A normal waveform with no secondary spark indicates plug/ignition gap/boot problem. A collapsed or distorted waveform points to driver or coil internal issues.

7) Remove coil (safe, ordered)
- Safety: engine cool, ignition OFF, key out. Disconnect negative battery if you want total isolation.
- Removal steps: unplug the coil connector, remove the mounting bolt(s), pull the coil straight out (quarters-turn/rock if stuck). For coil-on-plug, remove coil directly; for coil packs, remove pack assembly.
- Why/theory: careful removal prevents damage to the coil boot, plug and cylinder head sealing surfaces.

8) Inspect and service the spark plug
- What: inspect the plug for gap, electrode wear, deposits; replace if worn or fouled. Set correct gap.
- Why/theory: a worn or fouled plug increases required voltage; a coil may appear weak when the real problem is the plug. New coil onto a worn plug still misfires.

9) Install coil (in order)
- What: fit new or tested coil, ensure boot seats on plug, apply a very small amount of dielectric silicone grease to the inside of boot (not on electrode), torque mounting bolts to spec, reconnect coil connectors, reconnect battery if removed.
- Why/theory: clean seating ensures good electrical contact and prevents arcing. Proper torque prevents movement and improves heat dissipation.

10) Post-repair verification
- What: clear codes, start engine, check for misfire and live data. Run a road test and re-scan.
- Why/theory: ensures the repair fixed the fault and that no secondary damage (e.g., fouled cat) is causing symptoms.

How replacing the coil fixes the fault (theory, in order)
- Fault mode examples:
- Open primary winding → no current flow → no magnetic field → no induced high voltage → no spark.
- High-resistance primary/secondary → reduced current/induced voltage → weak spark → partial burn/misfire.
- Broken insulation/carbon tracking → internal leakage or external arcing → inconsistent/high-voltage escape → misfire and ECU-detected irregularities.
- Thermal/intermittent internal connection → misfires under load or heat-soak.
- Replacing the coil restores correct winding continuity and insulation. That re-establishes the proper primary current ramp and the resulting high-voltage collapse across the secondary to produce a strong, consistent spark at the correct timing. A good spark completes combustion properly, resolving misfire codes, smoothing idle, restoring power/efficiency and stopping unburned fuel from damaging the catalytic converter.

Quick notes and cautions (concise)
- Always verify supply voltage and driver signals before ordering coils — sometimes harness or ECU driver faults cause coil failure.
- Replace worn spark plugs when fitting new coils.
- Use OEM or quality aftermarket coils; improper specs can cause further problems.
- High-voltage can shock you; don’t probe secondary outputs with your fingers or uninsulated tools while cranking.

That is the ordered procedure with the underlying theory and how the repair cures the fault.
rteeqp73

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