Let’s face it: cars can be drama queens. One day they’re purring like kittens, the next they’re coughing, sputtering, or guzzling gas like there’s no tomorrow. If your ride’s been acting moody lately, a failing ignition coil might be to blame. These unsung heroes of your engine’s ignition system are easy to overlook—until they start causing chaos. Let’s break down the symptoms of a coil going bad, why they happen, and what you can do about it.
What Does an Ignition Coil Even Do?
Think of your ignition coil as the middleman between your car’s battery and its spark plugs. It transforms the battery’s low voltage (12 volts) into a jolt of up to 45,000 volts to create the spark that ignites the air-fuel mixture in your engine’s cylinders. Without this spark, your car wouldn’t start, period.
Most modern cars use a “coil-on-plug” system (one coil per spark plug), while older models might have a single coil feeding multiple plugs. Either way, a faulty coil can throw your engine’s rhythm out of whack.
Symptoms of a Coil Going Bad: Don’t Ignore These Red Flags
1. Poor Fuel Economy ⛽
Suddenly spending more time at the gas pump? A weak ignition coil can cause incomplete combustion, meaning your engine burns fuel less efficiently. To compensate, it might dump extra fuel into the cylinders, wasting gas and money. According to the Society of Automotive Engineers, misfires from bad coils can reduce fuel efficiency by up to 30%.
What to watch for:
- Your “miles per gallon” drops noticeably.
- You’re filling up more often, even with your usual driving habits.
2. Misfiring Engine 🔥
A misfire feels like your engine is hiccuping—jerking, sputtering, or even stalling at stoplights. This happens when the coil fails to send enough voltage to the spark plug, leaving unburned fuel in the cylinder. Over time, misfires can damage your catalytic converter (a very expensive fix).
Pro tip: Use an OBD2 scanner (like this one from AutoZone) to check for trouble codes like P0300 (random misfire) or P0351 (coil circuit malfunction).
3. Difficulty Starting 🚗💨
Ever been stuck in a parking lot, turning the key repeatedly while your car groans but won’t start? A dying coil might not generate enough spark to ignite the fuel, especially in cold or damp weather. If this happens alongside misfires, the coil is likely the culprit.
Other Sneaky Symptoms You Might Miss
- Check Engine Light 🚨
Your car’s computer is smarter than you think. It’ll often detect coil issues and trigger the check engine light. Don’t ignore it—90% of the time, this light signals a problem that needs fixing (Cars.com). - Backfiring or Rough Idling
Unburned fuel can ignite in the exhaust system, causing loud pops or bangs. You might also feel the steering wheel vibrate at stoplights. - Gas Smell or Fuel Leaks
A faulty coil can leave raw fuel in the exhaust, creating a gasoline odor. In rare cases, pressure buildup from misfires might even cause fuel leaks (Firestone Complete Auto Care). - Sluggish Acceleration
If your car feels like it’s dragging an anchor, weak sparks could be robbing your engine of power.
Why Do Ignition Coils Fail?
Ignition coils work hard—they handle extreme heat, vibration, and electrical stress. Common causes of failure include:
- Age: Most last 80,000–100,000 miles.
- Moisture: Leaky valve covers or car washes can fry coils.
- Faulty Spark Plugs: Worn plugs force coils to work harder.
How to Diagnose a Bad Coil (Without a Mechanic)
| Symptom | Quick Check |
|---|---|
| Misfires | Swap the suspect coil with another cylinder. If the misfire moves, replace the coil. |
| No Start | Test for spark using a spark tester (under $20 at AutoZone). |
| Check Engine Light | Scan for codes like P0351–P0358 (coil-specific faults). |
Can You Drive with a Bad Coil?
Short answer: Not for long. Driving with a faulty coil can:
- Damage your catalytic converter ($1,000+ to replace).
- Cause unburned fuel to wash oil off cylinder walls, leading to engine wear.
If you’re stranded, head straight to a mechanic—or use roadside assistance through your insurance.
Fixing the Problem: Coil Replacement 101
Replacing an ignition coil is a DIY-friendly job if you’re handy:
- Locate the coil (usually on top of the engine, attached to spark plugs).
- Unplug the electrical connector.
- Remove the mounting bolt, pull out the old coil, and insert the new one.
Costs range from 50–250 per coil, depending on your car. Brands like Bosch and Delphi offer reliable aftermarket options.
Not a DIYer? Sites like RepairPal can estimate local mechanic costs.
Prevent Future Coil Failures
- Replace spark plugs on schedule (check your owner’s manual).
- Fix oil leaks promptly—oil can degrade coils.
- Avoid cheap, off-brand coils.
The Bottom Line
Ignition coils are small but mighty. Catching issues early saves you from costly repairs and keeps your car running smoothly. If you’re spotting symptoms of a coil going bad—like misfires, poor gas mileage, or that dreaded check engine light—don’t wait. Test it, replace it, and get back to stress-free drives.
Got questions or a coil story to share? Drop a comment below!