Diagnostics · System playbook

Ignition System

Find the weak cylinder before you buy anything.

Modern coil-on-plug ignition rarely fails all at once — it fails one cylinder at a time, under load, when hot, or in the rain. The whole game is isolating WHICH cylinder and WHAT part: coil, plug, boot, or the wiring feeding it. Swap testing is free and beats guessing every time.

What the symptoms mean

Misfire under load only
Breaking down under cylinder pressure: plugs worn or coil boot arcing
Misfire when damp or after a wash
Cracked boots/wires tracking to ground — look at night for the light show
Single-cylinder misfire code (P030X)
That cylinder's coil, plug, injector, or compression — isolate before buying
Random multiple misfire (P0300)
Usually NOT ignition: think fuel supply, vacuum leak, or EGR first
Flashing check engine light
Active misfire dumping fuel into the cat — drive it as little as possible

The diagnostic order

Work top to bottom — the order is the point. Each step rules out the cheapest, most common causes before anything expensive gets blamed.

  1. Pull the misfire counters per cylinder from the scan tool. One cylinder = isolate it. All cylinders = stop, it is probably not ignition.
  2. Swap the suspect coil to another cylinder and clear codes. If the misfire follows the coil, done — that is a free, definitive test.
  3. If the miss stays put: pull the plug. Read it — oil fouling, carbon, closed gap, cracked porcelain all tell different stories.
  4. Boot and well check: oil in the spark plug well kills coils. If the well is wet, fix the valve cover gasket or the new coil dies too.
  5. Still there with known-good coil and plug? Compression or leak-down test that cylinder — ignition was never the problem.
  6. Check coil primary feed with the connector back-probed: power, ground, and trigger. Rodents and chafe points love coil harnesses.

Tools that matter here

Scan tool with per-cylinder misfire countersSpark plug socket + torque wrenchCompression testerMultimeter

The classic mistakes

  • Replacing all the coils for a single-cylinder miss. Swap test first — coils are individually diagnosable for free.
  • Chasing P0300 with ignition parts. Random-multiple is fuel, air, or mechanical more often than spark.
  • Installing plugs without torque — over-torqued plugs in aluminum heads become a much worse job later.
Pro tip: A misfire that returns after you swap the coil AND the plug is telling you to do a compression test. Believe it the first time — the parts cannon does not fix low compression.

Ignition System trouble codes (25)

P0300Random/Multiple Cylinder Misfire D…P0301Cylinder 1 Misfire DetectedP0302Cylinder 2 Misfire DetectedP0303Cylinder 3 Misfire DetectedP0304Cylinder 4 Misfire DetectedP0305Cylinder 5 Misfire DetectedP0306Cylinder 6 Misfire DetectedP0307Cylinder 7 Misfire DetectedP0308Cylinder 8 Misfire DetectedP0309Cylinder 9 Misfire DetectedP0310Cylinder 10 Misfire DetectedP0311Cylinder 11 Misfire DetectedP0312Cylinder 12 Misfire DetectedP0313Misfire Detected with Low FuelP0315Crankshaft Position System Variati…P0316Engine Misfire Detected on Startup…P2300Ignition Coil A Secondary Circuit …P2301Ignition Coil A Secondary CircuitP2302Ignition Coil B Secondary CircuitP2303Ignition Coil C Secondary CircuitP2304Ignition Coil D Secondary CircuitP2305Ignition Coil E Secondary CircuitP2306Ignition Coil F Secondary CircuitP2307Ignition Coil G Secondary CircuitP2308Ignition Coil H Secondary Circuit
Worked the whole playbook and it still isn't right? That's what the Diag Desk is for — a vetted master tech looks at YOUR readings on YOUR vehicle. $25, free if unanswered in 24h.