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Waveform Pattern

Ignition Waveform Diagnosis

Decode secondary ignition and coil-on-plug waveforms: firing line, spark line, coil oscillations, and current ramps. Learn how each shape points at a fouled plug, weak coil, open secondary, or lean cylinder.

Why the waveform beats the code

A P030x misfire code tells you which cylinder went bad; the ignition waveform tells you why. Same code, three completely different repairs: a worn plug, a shorted coil, or a lean-injector event all read "misfire on cylinder 4" to the PCM, but each produces a distinctly different scope pattern. Ten seconds with a scope saves an hour of parts-cannon guessing.

Anatomy of a secondary ignition waveform

Every spark event has three regions worth reading:

  • Firing line — the vertical spike at the moment of spark. Its height is the voltage required to jump the plug gap. Reflects gap size, cylinder pressure, and mixture.
  • Spark line — the flat (usually slightly drooping) plateau after the firing line. Its length is the burn duration; its height is the sustained arc voltage.
  • Coil oscillations — after the spark energy is used up, the remaining coil energy rings down through the primary. Three or more clean oscillations = healthy coil.

What you'll need

  • Lab scope with at least one channel
  • For distributor / wire-plug systems: capacitive secondary pickup (10 000:1 attenuation)
  • For COP systems: dedicated COP probe (paddle) or low-amp current clamp on the coil driver wire
  • Timebase of 2 ms/div for a single event; 20 ms/div for a parade of cylinders
  • Trigger on the rising edge of the firing line so the event lands mid-screen

Step-by-step capture

  1. Warm the engine to operating temperature — cold plugs read artificially high.
  2. Clamp or paddle the cylinder of interest. On COP, use the paddle over the coil body; on wire-plug, clip the capacitive pickup around the plug wire.
  3. Set timebase to 2 ms/div for a single event, 5–10 kV/div for secondary voltage.
  4. Idle steady and capture. Then snap-throttle to briefly load the cylinder — firing-line height should rise noticeably (10–15 kV → 20–25 kV on a healthy system).
  5. Compare cylinder to cylinder — the outlier is almost always the fault.

Reading the pattern

Healthy pattern

Firing line 8–15 kV at idle, spark line 0.8–2.0 ms at ~1–2 kV, three or more clean coil oscillations, return to zero volts baseline. Snap-throttle firing line climbs to 20–25 kV briefly, then settles.

Tall firing line, short spark line

The cylinder is running lean, the plug gap is too wide, or the plug electrode is worn. The coil had to build extra voltage to jump the gap, then the arc collapsed early. Confirm with a fuel-trim scan; if trims are normal, replace the plug.

Short firing line, long drooping spark line

The cylinder is rich, oil-fouled, or the plug is carbon-tracked. Low voltage jumped the gap easily, then the arc lingered as fuel kept re-igniting. Pull the plug and inspect — black wet = injector leak or ring wear; black dry = rich mixture or misfire.

Missing or damped coil oscillations

A shorted primary winding is dumping coil energy internally instead of ringing it down through the primary circuit. Replace the coil. This is the single most common COP failure mode after 60 000 miles.

Firing line off the top of the screen

Open secondary. The coil is trying to jump an infinite gap — usually a disconnected plug wire, a cracked COP boot arcing to ground, or a broken plug electrode. On a snap-throttle test the firing line hits the scope's clip limit (often 40+ kV) and stays there.

Firing line collapses to almost nothing

Shorted secondary — carbon-tracked plug, punctured coil boot arcing to the cylinder head, or a cracked plug insulator. The spark takes the path of least resistance to ground and never reaches the electrode.

COP primary current ramps

Coil-on-plug systems are easier to diagnose from the primary side. Clamp a low-amp probe around the coil driver wire (the switched-ground wire from the PCM) and read the ramp:

  • Healthy ramp: smooth climb to 5–9 A peak, brief hold, sharp collapse when the driver opens.
  • Stepped ramp: shorted turns inside the primary — replace the coil.
  • Low peak (<4 A): high primary resistance, a failing driver, or high circuit resistance.
  • No collapse: the PCM's driver never opened — driver failure or command signal issue.

Worked example: intermittent misfire on cyl 3

2013 V6, P0303 stored. Idle secondary capture on cyl 3 shows a firing line at 22 kV vs 12 kV on the other cylinders, spark line under 0.4 ms, and only two coil oscillations. Snap-throttle drives the firing line past 35 kV. Tall firing line + short spark line + reduced oscillations = worn plug on a tired coil. Replaced plug and coil on cyl 3, code cleared, misfire gone.

Common mistakes

  • Reading a cold-engine capture — firing lines run 20–30% high until warmup
  • Skipping the snap-throttle — many faults only appear under momentary load
  • Not comparing cylinder to cylinder — absolute numbers vary by engine family
  • Assuming a bad coil without checking the plug first — new plugs are cheap insurance
  • Ignoring the coil-oscillation region — it's the single best indicator of coil health

Frequently asked questions

What voltage should the firing line reach on a healthy ignition system?

On a warm engine at idle, 8–15 kV is typical for most modern coil-on-plug systems and 10–20 kV for distributor-based secondary. Values above 25 kV under light load point to a worn plug, open plug wire, or a lean cylinder. Values under 5 kV point to a shorted plug or fouled electrode.

How long should the spark line be?

0.8 to 2.0 ms is normal at idle. Short spark lines (<0.5 ms) with a tall firing line mean the coil dumped voltage fast, usually because the mixture is lean or the gap is oversized. A long, drooping spark line usually means the mixture is rich or the plug is fouling.

What does the coil oscillation region tell me?

After the spark burn, the coil rings down the remaining energy through the primary circuit. You should see at least three clean oscillations. Fewer than three ringdowns, or a heavily damped waveform, means the coil is losing energy internally — usually a shorted primary winding.

Can I diagnose COP coils without a secondary probe?

Yes. Capture the coil's primary current with a low-amp clamp around the coil driver wire. A healthy ramp climbs smoothly to peak saturation current (5–9 A on most COPs), holds briefly, then collapses. A stepped, jagged, or low-peak ramp means shorted primary turns or a failing driver.

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