How to tell if an engine coil is bad

You can tell an ignition coil is failing by confirming misfire or coil-circuit trouble codes with a scan tool (P030x/P035x), noting symptoms like rough idle and loss of power, and validating with simple tests such as a coil “swap test,” a spark tester, and checks for proper power, ground, and control signals. Resistance checks can help on some designs, but modern coil-on-plug (COP) units are best diagnosed with scan data, a spark tester, or an oscilloscope.

What an ignition coil does—and why it matters

The ignition coil transforms 12V battery power into tens of thousands of volts to jump the spark plug gap at precisely the right time. A weak or dead coil starves a cylinder of spark, causing misfires, rough running, poor acceleration, and potential catalytic converter damage from unburned fuel. Because modern engines typically use one coil per cylinder (COP), you can often isolate the faulty cylinder quickly.

Signs your ignition coil might be failing

The following symptoms commonly point to a weak or failed ignition coil. While these overlap with other ignition and fuel issues, they’re a reliable starting point for suspicion.

• Rough idle, engine shake, or intermittent stumble—especially under load.
• Hard starting or extended cranking, particularly when hot or in damp weather.
• Loss of power, hesitation, or surging during acceleration.
• Poor fuel economy and a raw fuel smell from the exhaust.
• Check Engine Light with misfire codes (P0300 random, P0301–P0308 specific cylinders) or coil circuit codes (P0351–P0358 and higher on multi-cylinder engines).
• Audible “snapping” or visible arcing around a coil or plug boot in the dark.

Any one symptom alone isn’t definitive, but a combination—especially paired with a relevant code—strongly suggests coil trouble.

How to diagnose a bad ignition coil

This sequence minimizes guesswork, starting with data you can collect quickly, then moving to simple hands-on tests before advanced tools.

1. Scan for codes: note P030x (misfire) and P035x (coil circuit) and record freeze-frame data.
2. Check live data: misfire counters, fuel trims, O2 behavior; confirm the misfiring cylinder.
3. Inspect: look for cracked coils, oil in plug wells, loose connectors, damaged boots, or carbon tracking on insulators.
4. Verify coil power and ground: with key on, B+ at the coil (near battery voltage) and solid ground; confirm the ECU control pulse on the trigger wire.
5. Perform a swap test: move the suspect coil to another cylinder and see if the misfire follows.
6. Use a spark tester: check for strong, consistent spark; avoid pulling plug wires to “watch” for spark on modern systems.
7. Consider resistance checks (where applicable): primary/secondary values can expose an open/short on certain coil types.
8. If available, view coil patterns on an oscilloscope to assess dwell, firing voltage, and burn time.

By confirming the misfire, narrowing it to a cylinder, and proving the coil is the root cause (not the plug, injector, or wiring), you can decide confidently whether to replace the coil.

Using an OBD-II scanner

A basic scan tool can quickly separate ignition from fuel or mechanical issues. Here’s what to look for and how to interpret it.

• Codes: P030x pinpoints the cylinder; P035x implicates that cylinder’s coil circuit (primary/secondary). Multiple P035x codes often indicate a shared power/ground issue.
• Freeze-frame: note RPM, load, coolant temp—misfires that appear only hot or under load often point to heat-sensitive coil breakdown.
• Live misfire counters (or Mode $06$): rising counts on one cylinder during idle or snap-throttle correlate with coil issues.
• Fuel trims: rich trims during misfire can indicate unburned fuel (ignition fault); lean trims might push the coil harder but usually point elsewhere.

If the misfire follows the cylinder identified in codes and improves when you replace or swap the coil, the diagnosis is typically complete.

Multimeter tests and expected values

A digital multimeter can catch some coil faults, but note that many modern COP coils don’t show reliable resistance patterns. Use this as a screening tool, not a verdict.

• Power/ground check: with key on, the coil’s power feed should be near battery voltage; voltage drop to ground should be minimal under load.
• Control signal: a test light or scope on the control wire should show pulsing as the ECU commands dwell; a noid/test light can confirm switching.
• Resistance (older designs): primary often ~0.3–1.0 Ω; secondary frequently ~6k–15k Ω on legacy canister/coils. COP assemblies vary widely; many won’t match these ranges and can still be good.

If primary or secondary is open (infinite resistance) or obviously shorted, the coil is bad. Otherwise, normal resistance doesn’t guarantee the coil is healthy under heat and load.

Swap and spark tests

These quick checks are highly effective for COP and coil-pack systems and don’t require advanced tools.

• Swap test (COP/coil-pack): move the suspect coil to another cylinder; clear codes and drive. If P030x moves with the coil, replace it.
• Spark tester: connect an adjustable gap tester; crank/run the engine. A strong, blue spark that consistently jumps a 10–12 mm gap is typical; weak/orange or intermittent spark suggests a bad coil or feed issue.

Always test with a known-good spark plug or tester; pulling connectors to “force” a misfire can stress electronics and is unsafe.

Oscilloscope (advanced)

A lab scope reveals firing voltage, burn time, and coil oscillations. A healthy pattern shows a sharp firing line, stable burn time (typically 1.2–2.0 ms at idle), and consistent ringing. Excessive firing voltage with short burn time points to a wide plug gap or weak coil energy; erratic dwell or missing events can indicate control or internal coil faults. Comparing cylinders side-by-side is decisive.

Coil types and what changes in testing

Different ignition architectures call for slightly different approaches. Identifying your system helps you pick the most effective test first.

• Coil-on-plug (COP): one coil per cylinder. Best tests are swap, spark tester, and scan data; resistance checks are often inconclusive.
• Coil pack (waste-spark): one coil fires two cylinders. Misfires on paired cylinders can implicate a single coil tower; inspect boots and plug wires if used.
• Single “canister” coil with distributor: secondary resistance tests are more applicable; inspect cap/rotor and wires for carbon tracking or cracks.

Regardless of type, verify power, ground, and control signals before condemning the coil itself.

What causes coils to fail

Understanding the root cause prevents repeat failures after replacement.

• Heat and vibration: engine-bay heat cycles crack insulation and solder joints.
• Oil or coolant intrusion: leaking valve-cover gaskets flood plug wells, causing arcing.
• Excessive plug gap or worn plugs: higher firing voltage stresses the coil.
• Moisture and corrosion: water in boots/connectors promotes misfire and tracking.
• Poor grounds or low system voltage: increases coil duty cycle and heat.
• Aftermarket quality variance: some low-cost coils fail prematurely.
• Lean conditions or injector faults: can raise coil demand and heat.

If a coil fails early, inspect plugs, boots, gaskets, and charging system to address the underlying stressor.

Fixing the issue and preventing repeat failures

Once you’ve confirmed a bad coil, these steps improve reliability and reduce the chance of another misfire.

• Replace the faulty coil; consider replacing the spark plug and boot on that cylinder at the same time.
• Set correct plug gap and torque; apply a thin film of dielectric grease inside boots.
• Repair oil leaks into plug wells (valve-cover gaskets) and replace contaminated boots.
• Use OEM or high-quality coils, especially on engines known for coil sensitivity.
• Clear codes, verify with a road test and live misfire counters; confirm stable fuel trims.
• If multiple coils are failing, check alternator output, engine grounds, and relevant TSBs for your vehicle.

Addressing both the failed component and the conditions that damaged it helps protect your catalytic converter and restores smooth operation.

Safety notes

Ignition systems generate dangerous voltage; use safe practices during diagnosis and repair.

• Do not touch coils or plug wires with the engine running; use insulated tools.
• Use a proper spark tester; avoid pulling coils or wires to create an artificial gap.
• Disable fuel or ignition per service manual when performing cranking tests.
• Disconnect the battery before resistance checks; keep fuel vapors away from spark testing.
• Wear eye protection and gloves; secure loose clothing and hair.

Following basic precautions reduces risk to you and to sensitive engine electronics.

Summary

To tell if an engine coil is bad, confirm the misfire with a scan tool, identify the affected cylinder, and validate with a swap test, spark tester, and power/ground/control checks. Resistance measurements can flag obvious faults on some designs, but modern COP coils are best verified with scan data or, ideally, an oscilloscope. Replace the bad coil and related wear items (plug/boot), fix oil or moisture intrusion, and use quality parts to prevent recurrence.

How to tell if a motor coil is bad?

6 Signs of a Faulty Ignition Coil

1. Signs of a Faulty Ignition Coil: Check Engine Light.
2. The Engine Starts Misfiring.
3. The Engine Won’t Start Normally.
4. The Engine Guzzles Gas.
5. The Car Doesn’t Perform Well.
6. The Car Backfires.

What does a bad coil feel like?

It’s Difficult to Drive Without Misfiring. If you can no longer drive without your engine misfiring, then you might have a faulty coil. When your car misfires, it vibrates or shakes while idling, and while you’re driving, it feels jerky or sputtery.

How do I know if my engine coil is bad?

Symptoms of a bad ignition coil include the illuminated check engine light, rough or erratic engine idling, engine misfires, backfiring, difficulty starting the engine, reduced power and poor fuel economy, and even engine stalling. A foul smell of gas from the exhaust can also be a sign of unburned fuel due to a misfiring cylinder. These symptoms indicate the ignition system is not consistently creating the spark needed for combustion, which can lead to more severe engine damage if left unaddressed. 
Here are the common symptoms of a faulty ignition coil:

• Check Engine Light: The most common initial sign of an ignition coil problem is a glowing check engine light on your dashboard. 
• Engine Misfiring: The engine may cough, sputter, or vibrate due to a cylinder not firing correctly because the ignition coil isn’t providing enough voltage for the spark plug. 
• Rough Idling: The engine’s smooth rumble at idle will turn into a juddering, faltering, and noisy experience, especially when the car is stationary. 
• Difficulty Starting: A failing ignition coil can make it harder for the engine to start, requiring the ignition system to work overtime. 
• Engine Stalling: In severe cases, a misfire from a faulty ignition coil can cause the engine to stall while driving. 
• Reduced Power and Fuel Economy: You might notice a significant loss of power and slower acceleration because the engine isn’t combusting fuel efficiently. 
• Backfiring and Gas Smell: When fuel doesn’t ignite in a cylinder, it can exit the exhaust and ignite, causing loud pops (backfiring) and a strong smell of gas. 

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.