📋 Quick Facts
Time
15 minutes
Difficulty
Easy
Tools
2 tools
Cost to DIY
$20-40
Modern engines use one coil per cylinder (coil-on-plug). When one starts to fail, you get a misfire code (P0301 for cylinder 1, P0302 for cylinder 2, etc.). The misfire moves with the coil if you swap it - that is the easiest test.
🛠 What You'll Need
- Digital multimeter with ohms scale (shop a digital multimeter on Amazon)
- Spark tester or known-good spark plug (shop a spark tester on Amazon)
- Safety glasses (shop safety glasses on Amazon)
Product links above are Amazon affiliate links. AmpAuto earns a small commission at no extra cost to you.
⚠ When NOT to DIY thisCoils generate 30,000+ volts. Never hold a disconnected plug wire or coil tower while cranking - the shock is painful and can knock you off a ladder. Always wear insulated gloves when handling coils with engine off.
🎯 Expected Readings (Pass/Fail Reference)
| Primary winding resistance | 0.4 - 2.0 ohms (check service manual for exact) |
| Secondary winding resistance | 6,000 - 15,000 ohms (6 - 15 k-ohms) |
| Open circuit on either winding | Infinity (OL) = bad coil |
| Spark test | Bright blue spark across 1/4" gap. Yellow/orange/weak = bad coil |
Numbers are typical. Always cross-check against your factory service manual for the exact spec.
📝 Step-by-Step Test Procedure
- Pull the trouble code firstScan with an OBD2 reader. Codes P0300 (random misfire) or P0301-P0308 (specific cylinder) point you to the right coil. No code? A scan tool with live data can show "cylinder x misfire counter" climbing.
- Cylinder swap test (best test)With engine cold and off, swap the coil from the suspect cylinder with one from a known-good cylinder. Clear codes, drive 5 minutes. If the misfire code moves to the new cylinder, the coil is confirmed bad. If it stays put, the problem is the spark plug, injector, or compression.
- Remove the suspect coil and inspectLook for cracks in the coil boot, oil contamination (head/valve cover leak soaking the coil), or burned electrodes. Oil-soaked coils almost always fail within months.
- Primary resistance testSet multimeter to ohms (200 range). Touch probes to the two small pins inside the coil connector. Should read 0.4 - 2.0 ohms. Infinity or short to ground = bad.
- Secondary resistance testSet multimeter to 20 k-ohms. One probe in the spark plug boot terminal, the other on a primary pin (typically the + terminal). Should read 6,000 - 15,000 ohms. Out of range = bad.
- Spark gap testPlug a spark tester into the coil boot, clip the tester ground to engine metal. Crank engine 2 seconds. Spark should be a bright blue arc across at least a 1/4" (6 mm) gap. Weak yellow/orange = failing coil.
- Check the coil control wire (advanced)Back-probe the trigger wire at the coil connector with multimeter on DC volts. Cranking: should pulse 0-12 V rapidly. No pulse = ECU or wiring problem, not the coil.
- Replace and clear codesIf confirmed bad, replace. On high-mileage engines (over 100,000 miles), replace the spark plug at the same time. shop ignition coils on Amazon.
✅ Pass / Fail Criteria
✅ PASS
Primary 0.4-2 ohms, secondary 6-15 k-ohms, bright blue spark at 1/4" gap, boot clean and uncracked
❌ FAIL
Out-of-spec resistance, infinity reading, weak yellow spark, cracked boot, oil-soaked, or misfire code follows it when swapped
🔧 If It Fails - What To Do Next
Replace the bad coil. Coils run $20-100 each (OEM is more reliable than the cheapest aftermarket). If multiple coils have failed within 50,000 miles, suspect a valve cover gasket leak dripping oil onto them. See Why is my check engine light on? and Why is my car shaking?
🔗 Related Guides
❓ Frequently Asked Questions
Can I drive with a bad coil?
For very short trips, yes, but a flashing check engine light from a bad coil dumps raw fuel into the catalytic converter and can destroy it within 50 miles.
Why does the cylinder swap test work?
If the misfire moves when you move the coil, the coil is bad. If it stays, the problem is the cylinder itself (plug, injector, compression).
How long should an ignition coil last?
80,000 - 150,000 miles. Heat, vibration, and oil contamination shorten life.
What ohm reading means a coil is bad?
Primary outside 0.4-2 ohms or secondary outside 6-15 k-ohms (always check the service manual for your specific car).
Should I replace all coils at once?
If one fails on a high-mileage engine and the others are original, replacing them all saves labor. Otherwise, replace one at a time.
Can a bad spark plug damage a coil?
Yes - a worn plug with a wide gap forces the coil to fire harder, which overheats the windings. Replace plugs at the recommended interval.