Typically, you should not drive far with a bad spark plug. A misfiring cylinder can damage the catalytic converter and engine, reduce power and fuel economy, and create safety risks. If you’re on the road, pull over safely and plan to get the vehicle inspected and repaired as soon as possible.
Understanding the risk
A spark plug that fails to fire can cause the affected cylinder to underperform or stop firing altogether. Prolonged misfires can overwork other ignition components, unburned fuel can wash down the cylinder walls, and unburned fuel entering the exhaust can overheat and damage the catalytic converter. The exact risk depends on your vehicle and how badly the plug is failing, but the potential for costly damage makes prompt attention prudent.
Symptoms of a failing spark plug
Common signs you may need a plug replacement include the following:
- Engine misfire or rough running, especially at idle or under load
- Loss of power or sluggish acceleration
- Decreased fuel economy or higher emissions
- Check Engine Light with misfire-related codes (for example P0301–P0306)
- Difficulty starting or rough cold starting
Observing these symptoms should prompt a diagnostic check to confirm whether the spark plug is the cause and to prevent further damage.
Safety and driving considerations
Before driving further with a suspected misfire, consider these guidelines:
- Do not drive long distances if the misfire persists or the engine runs very rough.
- If the check-engine light is flashing, stop safely and arrange for towing; a flashing light indicates a severe misfire risk to the catalytic converter.
- Drive slowly and avoid high speeds or heavy acceleration if you must move the car to a repair facility.
- Avoid high loads (towing, steep climbs) until the issue is diagnosed and repaired.
These steps help minimize the risk of engine damage and costly repairs.
When to pull over immediately
If you notice smoking, a strong fuel smell, engine knocking, or the vehicle running very poorly, pull over safely and seek roadside assistance. These conditions can indicate dangerous engine operation that requires immediate attention.
What to do if you suspect a bad spark plug
Follow these practical steps to diagnose and fix the issue efficiently:
- Check for diagnostic trouble codes (DTCs) with an OBD-II scanner to identify misfire-related codes.
- Inspect the suspect spark plug and its boot/coil for fouling, wear, or damage.
- Replace the faulty plug (and any worn or damaged ignition components) with the correct type and gap according to the manufacturer’s specifications.
- Reinstall or replace spark plug wires or coil boots if necessary, and torque to spec.
- Clear codes, then perform a test drive to confirm proper operation and check for residual misfires.
After replacement, monitoring the vehicle ensures the issue is resolved and prevents further damage.
Maintenance and replacement intervals
Typical replacement intervals vary by plug type and vehicle, but the general guidelines are:
- Copper plugs: approximately every 30,000–40,000 miles (48,000–64,000 km)
- Platinum plugs: approximately every 60,000–100,000 miles (96,000–160,000 km)
- Iridium plugs: approximately every 100,000 miles (160,000 km) or more
- Always follow the vehicle’s OEM specification and use the correct gap and torque; do not reuse damaged plugs.
Regular inspection of ignition components and adherence to manufacturer guidelines help prevent failures and protect the exhaust system.
Summary
Driving with a bad spark plug is generally not advisable, especially for long distances. A misfire can reduce performance, worsen fuel economy, and risk damage to the catalytic converter and engine. If you notice symptoms, have the vehicle diagnosed promptly and replace the faulty plug and related ignition parts as needed. With timely maintenance, most cars can continue to operate reliably after a plug is replaced.