What are the symptoms of a bad spark plug wire?

Symptoms of a failing spark plug wire typically include rough idle, engine misfires, and reduced power. In many cases, you’ll notice worse performance under load or during acceleration before a cable fails completely.

Common symptoms motorists notice

The following signs appear most often when a spark plug wire is deteriorating or has an open circuit or short to ground. If you notice several of these together, it’s a strong hint the wires may be at fault.

• Rough idle and engine misfires


• Hard starting or intermittent starting problems


• Loss of power and reduced acceleration or surge in hesitation


• Increased fuel consumption and higher exhaust emissions


• Check engine light or misfire trouble codes (for example P0300 and cylinder-specific P0301–P0306)


• Backfiring or popping through the intake or exhaust


• Visible damage to the wires or boots (cracked insulation, melted boot, exposed conductor, brittleness)


• Wires feel hot or emit a faint electrical odor under load

The above signs often point to wear or damage in the wire insulation, moisture intrusion, or a weakening inner conductor that allows current to leak or arc.

Less obvious or intermittent symptoms

Some symptoms show up only under certain conditions, making diagnosis trickier. Consider these if the main signs aren’t always present.

• Intermittent misfires that occur under specific temperatures, humidity, or fuel conditions


• Misfires that occur only at certain RPM ranges or during heavy acceleration


• Electrical interference, unusual radio static, or a noticeable arcing sound near the spark plug boots


• Moisture or rain exposure causing brief improvements or sudden worsening

Intermittent signs can also result from aging wires that conduct poorly but aren’t visibly damaged yet, or from related issues such as worn boots or a failing coil. Replacing worn wires is common in such cases.

What to do if you suspect a bad spark plug wire

Visual inspection

Start with a safe, cold engine. Look for cracked, brittle, or swollen insulation and damaged connectors at the spark plug boots. Look for signs of arcing along the length of the wire or at the boots.

Testing and replacement considerations

For a more definitive check, use a multimeter to check continuity and resistance per wire, if your vehicle’s service manual provides acceptable ranges. A spark tester or temporary swap with a known-good wire of the correct length and type can confirm a problem. If wear is evident or resistance tests show high values or open circuits, replace the wires. In many vehicles, it’s best to replace spark plug wires as a set to ensure even performance across all cylinders, especially on multi-wire ignition systems.

Summary

Bad spark plug wires typically reveal themselves through rough idle, misfires, reduced power, and poorer fuel economy, often accompanied by visible damage or electrical symptoms. If you notice several of these signs, inspect the wires, perform basic testing, and consider replacement. Replacing worn wires can restore smooth running, improve efficiency, and help prevent further engine issues.

How does a car act when spark plugs are bad?

A car with bad spark plugs will exhibit symptoms like rough idling, difficulty starting, poor acceleration, and decreased fuel economy. You might also notice engine misfires, a "check engine" light, or strange engine noises like knocking or rattling.
 
Common symptoms of bad spark plugs

• Rough idling: The engine may shake or feel unsteady, especially when the car is stopped. 
• Difficulty starting: The engine may crank for a longer time before starting, or require the throttle to be depressed. 
• Poor acceleration: The car may feel sluggish or "jerky" when you press the gas pedal. 
• Decreased fuel efficiency: You will likely notice you need to fill up your gas tank more often because the engine is burning fuel inefficiently. 
• Engine misfires: This can feel like an abrupt jerk or stutter and is a clear sign that a spark plug isn't firing correctly. 
• "Check Engine" light: Modern cars can detect a misfire and trigger the "check engine" light, which may be accompanied by a specific misfire code. 
• Engine knocking or rattling: A bad spark plug can cause an unusual knocking sound, particularly during acceleration. 
• Increased emissions: Incomplete combustion from faulty plugs can lead to higher levels of harmful emissions. 

This video explains the symptoms of bad spark plugs: 59shaile car care YouTube · Apr 23, 2023
What to do
If you experience these symptoms, it's important to have your spark plugs checked, especially if they are past the manufacturer's recommended replacement interval. Continued driving with bad spark plugs can lead to further engine damage. 
You can watch this video to learn how to identify bad spark plugs: 58sEasyAutoFixYouTube · Mar 5, 2023

How do I tell if my spark plug wires are bad?

Oh there we go. Yeah you see it oh right there. There we go there's our misfire. As I spray it the engine is shaking. More.

Does changing spark plug wires make a difference?

While they rarely require attention, spark plug wires do eventually need replacing, otherwise your engine performance will suffer. Ignoring the signs of worn-out spark plug wires can lead to decreased fuel economy, unstable idling, and even engine failure.

What are the symptoms of a bad ignition coil wire?

Symptoms of a bad ignition coil wire include a rough or sputtering engine, difficulty starting, a Check Engine light, and poor performance, such as sluggish acceleration and lower fuel economy. You may also notice the car stalling, a loss of power, or even loud backfiring from the exhaust, which can be accompanied by a strong fuel smell.
 
Engine performance and starting issues 

• Engine misfires: The engine may cough, sputter, or shake because a cylinder isn't firing properly. 
• Rough idle: The engine may shake or run unevenly, especially when stopped. 
• Difficulty starting: The engine may crank longer than usual or fail to start, especially in cold weather. 
• Stalling: The vehicle may stall while driving or idling. 
• Sluggish acceleration: The car may feel sluggish and lack power when you press the gas pedal. 

Warning lights and sounds 

• Check Engine light: The light will often illuminate or start flashing when an ignition coil is failing. 
• Exhaust backfiring: You may hear loud popping or banging sounds from the exhaust. 
• Fuel smell: A strong smell of gasoline may be present, especially when the engine backfires.