How to Tell If an Ignition Coil Is Bad

The fastest way to tell if an ignition coil is failing is to look for a rough idle or hesitation under load, a Check Engine Light with cylinder-specific misfire codes (P0301–P0308) or coil circuit codes (P0351–P0358), and to confirm by swapping coils, using a spark tester, or checking misfire counters with a scan tool. In practice, a bad coil often shows up as a misfire that gets worse when hot, under acceleration, or on hills; targeted tests under load are more reliable than static resistance checks. Below is a clear guide to symptoms, simple checks you can do, professional diagnostics, how to differentiate coil failure from other causes, and what to do next.

What a Bad Ignition Coil Looks and Feels Like

Ignition coils convert battery voltage into the high voltage needed to fire spark plugs. When they begin to fail, the signs are fairly consistent and often worsen as the coil heats up. The following list outlines the most common symptoms drivers and technicians observe.

• Engine misfire: shaking at idle, stumble on acceleration, or a “chug” under load
• Check Engine Light: P0300 (random misfire) or P0301–P0308 (misfire on a specific cylinder); P0351–P0358 for coil primary/secondary circuit faults
• Poor acceleration, reduced power, and sluggish throttle response
• Hard starts or no-start, especially when hot (hot-soak failure)
• Rough idle that smooths out at higher RPM, or vice versa
• Backfiring or catalyst-damaging raw fuel smell from the exhaust
• Decreased fuel economy
• Audible snapping/ticking if arcing from a cracked coil or boot in the dark

One or more of these symptoms—especially paired with a matching diagnostic code—strongly points to an ignition coil problem, though other faults can mimic these signs.

Quick Checks You Can Do First

You don’t need a full toolbox to narrow this down. These basic steps can quickly confirm whether a coil is the likely culprit and are commonly used by technicians as first-pass diagnostics.

1. Visual inspection: Look for cracked housings, melted plastic, oil in the spark plug wells (leaking valve cover gaskets), corroded connectors, or torn coil boots.
2. Scan for codes: Use an OBD-II scanner to read P030x (which identifies the cylinder) and P035x (coil circuit) codes; note freeze-frame data such as engine load and temperature.
3. Coil swap test (COP systems): Move the suspected coil to another cylinder, clear codes, and drive. If the misfire code follows the coil (e.g., P0302 becomes P0304), the coil is bad.
4. Spark test: Use an inline or adjustable-gap spark tester. A strong, consistent blue spark across a 10–12 mm gap is expected; weak/orange or intermittent spark suggests coil or boot issues.
5. Plug check: Remove the spark plug from the misfiring cylinder; excessive gap, worn electrodes, oil fouling, or carbon tracking on the ceramic can implicate the coil and/or plug.

If these steps point to the coil, you can be reasonably confident without deeper teardown. If not, continue with more advanced diagnostics below.

Deeper Diagnostic Tests (Shop-Level and DIY With Tools)

OBD-II data and misfire counters

A capable scan tool can display live misfire counters by cylinder and Mode $06 test results. Persistent counts on one cylinder under load (climb, acceleration) indicate ignition stress. Coil circuit codes (P0351–P0358) specifically implicate the coil or its wiring. Always note freeze-frame data to replicate the conditions that triggered the fault.

Multimeter resistance checks (limited but sometimes useful)

With the engine off and the coil disconnected, you can measure coil resistance, but modern coil-on-plug units often pass static tests even when bad. As a rough guide, primary resistance is typically around 0.3–2.0 ohms, and secondary often 5,000–15,000 ohms—check the service manual for your vehicle. Infinite or wildly out-of-spec readings can confirm failure, but “in-spec” does not guarantee a good coil under load.

Coil output with an adjustable spark tester

Set an adjustable tester to a 10–12 mm gap, ground it properly, and crank or run the engine. A strong, rhythmic blue spark indicates adequate coil output. Weak, erratic, or no spark points to a failing coil, a bad boot, or a wiring/driver issue. Avoid makeshift screwdriver-to-ground tests; they can damage electronics.

Oscilloscope testing (best but specialized)

An ignition scope can reveal the coil’s current ramp, dwell, and burn time. Typical burn time is around 1.3–2.0 ms; a short or erratic burn time, excessive firing KV, or an uneven current ramp can indicate internal coil breakdown, shorted windings, or secondary leakage. This test distinguishes an ignition fault from lean mixtures or compression problems, which produce different patterns.

How to Tell It’s the Coil—and Not Something Else

Ignition coils are part of a system. The following are common impostors that can cause similar misfire symptoms. Checking them helps avoid replacing a good coil.

• Spark plugs: Worn or over-gapped plugs increase coil load and mimic coil failure.
• Boots and wires: Carbon tracking, tears, and moisture cause misfire and arcing.
• Fuel injectors: A dead or clogged injector will cause a cylinder-specific misfire without coil-related codes.
• Vacuum leaks or intake gasket leaks: Often cause lean misfires at idle on specific banks.
• Low compression: Burned valves or ring issues produce persistent misfires unaffected by coil swaps.
• Crankshaft/camshaft sensors and timing issues: Can cause random misfires (P0300) across cylinders.

If swapping coils moves the misfire, it’s ignition-side. If it stays on the same cylinder after a coil swap, investigate the plug, injector, compression, or wiring.

Why Coils Fail—and How to Prevent It

Ignition coils are stressed by heat, vibration, and excessive voltage demand. Preventive maintenance can extend their life and reduce repeat failures.

• Over-gapped or worn spark plugs force coils to work harder; replace plugs on schedule.
• Oil in plug wells from valve cover leaks degrades boots and shorts the spark.
• Heat and age: Under-hood temps and thermal cycling break down insulation over time.
• Moisture and corrosion: Water intrusion or road salt can crack housings and corrode terminals.
• Electrical stress: Weak batteries, poor grounds, or charging system spikes can stress “smart” coils.
• Low-quality parts: Cheap coils often fail prematurely; OE or reputable aftermarket brands last longer.

Address underlying causes—especially plug condition and oil leaks—when replacing a coil to avoid repeat failures.

Replacement Tips, Costs, and Best Practices

Coil replacement is usually straightforward on coil-on-plug engines. Costs vary by vehicle and part quality. Consider these practical pointers before you begin.

• Typical parts cost: $25–$120 per coil (COP), $80–$300 for older multi-tower coil packs.
• Labor time: About 0.3–1.0 hour per accessible coil; more if intake components must be removed.
• Replace spark plugs with the coil if they’re due; an old plug can kill a new coil.
• Use dielectric grease inside boots and torque plugs to spec to prevent arcing and ease future service.
• Replacing all coils? Not required. Replace the failed coil and any obviously weak ones; consider sets only on high-mileage vehicles with repeated failures.
• Clear codes and verify with a road test under similar load and temperature conditions.

For most vehicles, a single bad coil can be replaced in under an hour; confirming the fix with a test drive prevents comebacks.

Safety Notes

Ignition systems generate high voltage. Work with insulated tools, keep hands clear of live components, and avoid open sparks near fuel vapors. Don’t unplug coils or injectors on a running engine to “find” a misfire—doing so can damage the catalytic converter or electronics.

Is It Safe to Keep Driving?

If the Check Engine Light is flashing, the misfire is severe enough to damage the catalytic converter—stop driving and address it immediately. A steady light with mild symptoms may allow short trips, but continued driving risks costly repairs and unsafe operation.

Summary

A bad ignition coil typically reveals itself through misfires, power loss, and Check Engine Light codes like P030x or P035x. Confirm by scanning for misfire data, swapping coils to see if the misfire moves, and using a spark tester; static resistance checks are less definitive on modern coils. Rule out worn plugs, damaged boots, injectors, and vacuum leaks, fix root causes such as oil intrusion, and replace the affected coil with quality parts. Prompt repair prevents catalytic converter damage and restores performance and efficiency.

How can you tell if you need an ignition coil?

You might need new ignition coils if your car displays symptoms like engine misfires, making it jerk or stall, a rough or shaky idle, poor fuel economy, a noticeable loss of power, difficulties starting, or a sputtering/coughing engine sound. An illuminated check engine light is also a strong indicator that a fault has been detected by the vehicle’s computer, often due to one of these misfires.
 
Common Symptoms of Bad Ignition Coils

• Engine Misfires/Rough Idling: You may feel a noticeable jerk, stutter, or shaking, especially when accelerating or at a stop. 
• Loss of Power: The engine may feel sluggish, and acceleration can be slow or weak. 
• Poor Fuel Economy: Your car might consume more gas than usual, as inefficient combustion doesn’t utilize fuel effectively. 
• Difficulty Starting: The engine might crank for a long time before starting, or it may refuse to start at all. 
• Backfiring: Loud popping or banging noises from the exhaust can occur when unburned fuel ignites in the exhaust system. 
• Check Engine Light: A faulty coil can lead to a misfire, which the engine’s computer detects, triggering the check engine light. 
• Strange Engine Noises: Listen for coughing, sputtering, or other unusual sounds from the engine. 

This video explains what a bad ignition coil does to your engine and how to tell if yours is failing: 1mAuto V Fix YouTube · Jun 13, 2025
What to Do if You Suspect a Bad Coil

• Don’t delay: Continuing to drive with a bad ignition coil can damage other components, such as the catalytic converter. 
• Scan for codes: Use an OBD-II scanner to read specific error codes (like a P030X code for a misfire) that can pinpoint the issue. 
• Consult a mechanic: A professional technician can accurately diagnose the problem, test the ignition coil, and recommend replacement. 

Can coil cause car jerking?

You Can’t Get Your Car to Run Smoothly
This makes your car shake while idling and jerk or sputter while you’re driving. A bad coil can cause your engine to misfire, and the only solution is to get that coil replaced.

How do you test the ignition coil?

To check an ignition coil, you can perform an ohm test using a multimeter to check primary and secondary winding resistance against a service manual, use a spark tester to look for a strong, blue spark, or conduct a power balance test by disconnecting each coil while the engine is running and listening for an RPM drop. A strong, bright, blue spark is good, while no spark, weak, or orange/red, zigzagging spark indicates a faulty coil.
 
Methods for Checking an Ignition Coil

1. Ohm Test (Multimeter Method)
   • Purpose: Measures the electrical resistance of the coil’s windings to see if they are within the manufacturer’s specifications. 
   • Steps:
     1. Set a multimeter to the resistance (Ohms, Ω) setting. 
     2. Locate the terminals on the coil and consult your vehicle’s service manual to find the correct points to test for the primary and secondary windings. 
     3. Connect the multimeter probes to these points. 
     4. Compare the reading to the specifications in your service manual. 
   • Interpretation:
     • Good Coil: The resistance reading is within the specified range. 
     • Bad Coil: A reading of zero, excessively high resistance, or a reading outside the specifications means the coil is faulty and needs to be replaced. 
2. Spark Tester (Visual Spark Method)
   • Purpose: Visually confirms if the coil is producing a strong, proper spark. 
   • Steps:
     1. Connect the spark tester’s lead to the ignition coil, or the vehicle’s secondary lead to a spark plug. 
     2. Connect the tester to a good ground point on the engine. 
     3. Crank the engine. 
   • Interpretation:
     • Good Coil: You will see a strong, vibrant, blue spark jumping the gap of the tester. 
     • Bad Coil: No spark or a weak, orange-red, or zigzagging spark indicates a problem with the coil. 
3. Power Balance Test (Engine Performance Method)
   • Purpose: Quickly identifies which cylinder’s coil may be failing by observing the engine’s reaction when a coil is momentarily disabled. 
   • Steps:
     1. With the engine running, briefly disconnect each ignition coil one by one. 
     2. Listen and feel for a noticeable change in the engine’s sound and performance. 
   • Interpretation:
     • Good Coil: You will feel a distinct decrease in engine RPMs and the engine will run rougher. 
     • Bad Coil: If you disconnect a coil and there is little to no change in the engine’s performance, that coil is likely the cause of a misfire. 

What are the symptoms of a failing ignition coil?

Symptoms of a failing ignition coil include a Check Engine Light, engine misfires and rough idling, stalling, loss of power, poor fuel economy, difficulty starting the engine, and exhaust backfiring or a smell of gas. These issues occur because the coil fails to deliver enough voltage to the spark plugs, preventing proper combustion. 
Common Symptoms

• Check Engine Light: A blinking or steady Check Engine light often indicates a misfire, which is a common sign of a failing ignition coil. 
• Engine Misfires/Rough Idling: You may notice a jerky or stuttering motion while driving, or the engine may feel unstable and vibrate when idling. 
• Stalling: The engine might cut off or stall when you come to a stop. 
• Loss of Power: The vehicle may feel sluggish, especially during acceleration, due to incomplete combustion in the cylinders. 
• Poor Fuel Economy: With less efficient combustion, your car will consume more fuel, leading to lower gas mileage. 
• Difficulty Starting: The engine may crank longer or fail to start altogether. 
• Exhaust Backfiring: You might hear a sputtering or coughing sound from the engine or tailpipe. 
• Smell of Gas: An incomplete combustion process can send unignited fuel into the exhaust, creating a gas smell. 

What’s Happening Inside the Engine
A failing ignition coil cannot provide the proper voltage to the spark plug. This leads to: 

• Incomplete Combustion: The fuel-air mixture in the cylinder doesn’t ignite correctly. 
• Misfires: The spark plug fails to do its job, causing the engine to misfire. 
• Engine Performance Issues: The misfires disrupt the smooth operation of the engine, resulting in the symptoms listed above.