Ignition coil inspection

Safety first

• Coils can generate tens of thousands of volts. Don’t touch the coil output or plug ends while cranking/running.
  
  

• Use an inline/adjustable spark tester—never hold a plug to ground by hand.
  
  

• If probing circuits, use proper back-probe pins and avoid shorting terminals.
  
  

Tools you’ll want

• OBD-II scan tool (live data, Mode $06 misfire monitors)
  
  

• Adjustable spark tester (0–30 kV)
  
  

• Digital multimeter (DMM), back-probe leads
  
  

• Low-amp clamp + oscilloscope (best) or LED logic probe for control signals
  
  

• Noid/test light (use only where appropriate—see notes)
  
  

• Feeler gauge for plug gap, dielectric grease, insulated pliers
  
  

• Heat gun / water mist bottle (to reproduce heat or moisture faults)
  
  

What are you common symptoms? 

• Rough idle, stumble on load, high fuel trims, poor economy, flashing MIL under acceleration
  
  

Quick reference (typical*—check service data for your car)

• Coil primary resistance (older/distributor or many waste-spark): 0.3–2.0 Ω
  
  

• Coil secondary resistance (older types): 6–15 kΩ
  
  

• COP coil resistance specs are often not reliable or not published—use functional tests.
  
  

• Coil current (many systems): 6–10 A peak; dwell 2–5 ms at idle (varies)
  
  

• Healthy spark demand: ~8–12 kV idle, 15–25 kV under load
  *Always prefer manufacturer specs if available.
  
  

The inspection workflow

1) Pre-checks (fast wins)

• Scan for codes: note P030X/P035X and freeze-frame conditions (RPM, load, temp).
  
  

• Open Live Data: look at misfire counters, short/long fuel trims, coolant temp (for heat-soak patterns).
  
  

• If your scanner supports Mode $06, check per-cylinder misfire data; identify the worst cylinder(s).
  
  

Decision: If one cylinder stands out → start there. If random → do global checks below.

2) Visual & easy mechanical checks (engine OFF)

Remove engine cover. Inspect coil(s) and boots:

4. • Cracks, melted spots, carbon tracking lines, swollen or oil-soaked boots.
     
     

   • Water in plug wells (common after washing), oil in wells (leaking valve cover seals).
     
     

Unplug each coil connector; check terminal fit/corrosion. Reseat each connector.

Pull the spark plug(s) from suspect cylinder(s):

4. • Gap within spec? Electrode worn/eroded? Cracks in insulator?
     
     

   • Oil or fuel fouling? Replace plugs if worn or fouled; set correct gap.
     
     

If you find carbon tracking or cracked boots: replace the boot (or coil if boot non-serviceable). Clean the well, install new plug, apply a tiny smear of dielectric grease inside the boot (not on the plug tip).

3) Power, ground, and control checks (KOEO / cranking)

Different coil types, different rules:

• 2-wire coils (older/waste-spark): one B+ feed, one ECM-switched ground.
  
  

• 3-wire “smart” COP coils: B+, ground, and a low-current logic control from ECM (often 0–5 V).
  
  

With Key ON, back-probe the coil B+ pin:

7. • Expect battery voltage (~12 V). If low/zero → check fuses/relay/wiring.
     
     

Ground integrity (engine cranking or running): do a voltage drop test:

7. • Meter on coil ground pin → battery negative; during dwell (or with scan-tool active test) drop should be ≤0.1 V. More = repair ground/wiring.
     
     

Control signal check:

7. • 2-wire coils: a regular incandescent test light from B+ to control should blink while cranking (ECM pulsing the ground).
     
     

   • 3-wire smart COP: do not use an incandescent test light on the logic wire. Use an LED probe or oscilloscope to see a square-wave control.
     
     

If B+ or ground is missing/weak → fix that before blaming the coil.

4) Functional spark output test (best quick diagnosis)

Install an adjustable spark tester on the suspect coil output (COP adapters are available).

10. • Start at ~10–12 kV gap; crank or run.
      
      

    • A strong, consistent spark that can jump 20–25 kV (briefly) is healthy.
      
      

    • Weak, intermittent, orange/yellow spark, or no spark → continue.
      
      

• Waste-spark coils: test both towers. A failure on one tower often means that whole coil section is suspect.
  
  

5) “Swap test” (COP & coil packs)

Move the suspect coil to another cylinder and move that cylinder’s good coil back.

    Clear codes; run the engine/road-test.

11. • If the misfire code follows the coil (e.g., from P0302 to P0304), the coil is bad.
      
      

    • If it stays on the original cylinder, look at plug, injector, compression, or wiring/ECM.
      
      

This is one of the highest-confidence, no-special-tool checks.

6) Resistance checks (only when specs exist)

Engine OFF, connector removed. On older coils/coil packs (not smart COP):

13. • Primary: measure between B+ and control pins → expect 0.3–2.0 Ω.
      
      

    • Secondary: between the output terminal(s) and B+ (or as specified) → expect 6–15 kΩ.
      
      

    • Open circuit or wildly out of spec → replace.
      
      

    • Note: Many modern COP coils do not have meaningful ohm specs. Use functional/scope tests instead.
      
      

7) Advanced: scope tests (best quality diagnosis)

Primary voltage waveform (2-wire): back-probe control, scope to chassis ground.

14. • See a dwell period (low), then a sharp flyback spike (~200–400 V) on release. Missing/low spike indicates coil or driver issue.
      
      

Coil current ramp (low-amp clamp on B+ feed):

14. • Smooth rising ramp to ~6–10 A with a flat top (saturation). A noisy/stepped ramp or early plateau → shorted turns; a very slow ramp → high resistance in feed/ground.
      
      

Secondary kV probe (if available):

14. • Compare cylinders. A single coil requiring unusually high kV suggests wide plug gap/lean mix/insulation leak; unusually low kV suggests weak coil/saturated plug.
      
      

8) Stress & intermittent tests

Heat-soak: warm the suspect coil gently with a heat gun; if misfire appears, coil insulation may be breaking down when hot.

Moisture test: light water mist around boots at idle (dark area). Listen/look for arcing; if it stumbles, replace boot/coil and clean carbon tracks.

Snap-throttle: watch misfire counters live; load-related misfire points to ignition breakdown under higher kV demand.

9) If the coil tests good, don’t forget

• Spark plug: wrong gap, cheap plugs, or extended-life plugs far past interval will kill coils.
  
  

• Valve cover leaks: oil in wells → fixes must include seals.
  
  

• Power/ground shared circuits: a bad splice can starve multiple coils.
  
  

• ECM driver: if no control on one cylinder and wiring is good, ECM may be at fault (rare; confirm carefully).
  
  

• Fuel/engine: injector, compression, vacuum leaks can mimic ignition faults.
  
  

Replacement & finishing notes

• Use OEM-quality coils—cheap ones often fail early.
  
  

• Replace any damaged boots and spark plugs together; set correct gap.
  
  

• Tiny dab of dielectric grease inside the boot (not on electrode).
  
  

• Torque coil hold-downs to spec (typ. 6–10 N·m; check your data).
  
  

• Clear codes and perform a road test; confirm no misfires under load and hot soak.