In short, you should not routinely drive with bad spark plugs in a gasoline-powered engine. Misfires can damage the catalytic converter, waste fuel, and eventually degrade engine performance. If you notice symptoms, limit driving and arrange repairs promptly.
Spark plugs ignite the air-fuel mixture in each cylinder. When they wear out or foul, the engine loses efficiency and runs poorly. This article explains what bad plugs look like, the risks of continuing to drive, how to respond, and how replacement works today.
What makes spark plugs go bad?
Common failure modes include worn electrodes, fouling from oil or fuel, overheating, and improper gaps. Understanding the causes helps you anticipate symptoms and plan maintenance.
Common causes
- Normal electrode wear leading to weaker sparks
- Oil fouling from worn piston rings or valve seals
- Fuel fouling from excessive fuel or poor combustion
- Overheating or incorrect heat range
- Coolant intrusion from a head gasket leak
Plugs typically last longer with modern materials: copper plugs may last 20,000–40,000 miles, while platinum or iridium plugs can last 60,000–150,000 miles depending on the vehicle and driving conditions. Always follow the manufacturer's recommended interval.
Signs that your spark plugs may be failing
Look for a combination of symptoms rather than a single telltale sign. If several symptoms appear together, plan a check or replacement soon.
- Rough idle or engine misfires
- Difficulty starting or intermittent starting problems
- Loss of power or sluggish acceleration
- Lower fuel economy and higher emissions
- Engine knocking or pinging, especially under load
- Illuminated check engine light
- Unusual engine noises or misfiring sounds
Driving with these symptoms can cause further damage, especially to the catalytic converter, which can be expensive to replace. If your engine is running rough, seek service promptly.
What to do if you suspect bad plugs
If you notice symptoms, here's a practical plan to minimize risk while getting the problem fixed.
- Limit driving to essential trips and avoid high loads or high speeds if possible.
- Check for diagnostic trouble codes with an OBD-II scanner or at a shop to confirm misfire codes (P0300–P0308).
- Replace spark plugs using the manufacturer’s recommended type, gap, and torque; replace ignition coils or spark plug wires if they show wear.
- Inspect the ignition system for corrosion, damaged boots, or worn cables; replace as needed.
- Test drive to verify improved running and confirm no remaining misfires.
After replacement, you should notice smoother idle, better acceleration, and improved fuel economy. If symptoms persist, a deeper ignition or fuel system diagnosis may be needed.
Replacement options and maintenance tips
Understanding what to expect helps manage cost and performance. Here are typical considerations when replacing spark plugs today.
- Platinum or iridium plugs last longer than copper but cost more upfront.
- Follow the specified gap and torque to avoid damage or poor performance.
- Replace worn ignition coils or wires if your vehicle uses a coil-on-plug or distributor system.
- Consider a complete tune-up or diagnostics if misfires recur after replacement, as underlying issues may exist (oil leaks, faulty seals, or ignition coil problems).
Prices vary by vehicle and region. Parts can range from $2–$20 per spark plug for copper to $10–$40 or more for platinum/iridium, with labor typically adding $50–$150 depending on engine layout and local rates. Some modern engines require specific OE plugs, so always consult the owner’s manual or a qualified technician.
Summary
Bad spark plugs degrade performance, fuel efficiency, and emissions, and can eventually cause engine damage if ignored. While you can technically drive a short distance with a mild misfire, continuing to drive with bad plugs risks costly repairs and breakdowns. Prioritize a timely inspection and replacement, and follow the manufacturer’s recommendations for type, gap, and replacement interval.