You can usually drive a few hundred miles on bad spark plugs, but the damage accelerates quickly

Bad spark plugs don't cause when ready engine failure, but they degrade performance fast and can cause expensive damage if you ignore them. Most drivers can make it to a repair shop within a few hundred miles, though the exact distance depends on how badly the plugs have worn and what kind of driving you're doing. City driving with frequent stops is harder on a struggling engine than highway cruising, so your mileage will vary.

The real risk isn't a sudden breakdown—it's that bad spark plugs force your engine to work harder, which damages the catalytic converter, fouls the oxygen sensors, and can eventually crack the engine block or warp the cylinder head. These repairs cost $500 to $2,500 depending on what fails. A set of spark plugs costs $50 to $200 installed, so the math is straightforward: replace them now or pay much more later.

Key Takeaways

  • Bad spark plugs let you drive a few hundred miles, but each mile increases the risk of damage to the catalytic converter and oxygen sensors.
  • Rough idle, misfiring, poor fuel economy, and hard starting are the main warning signs that spark plugs need replacement.
  • Spark plugs typically last 30,000 to 100,000 miles depending on the type—check your owner's manual for the interval your vehicle needs.
  • If you're delivering food and your check engine light is on, get the vehicle scanned at a shop before the next shift to avoid being stranded mid-delivery.

What happens when spark plugs wear out

A spark plug ignites the fuel-air mixture in each cylinder thousands of times per minute. As the electrode erodes, the gap widens and the spark weakens. The plug still fires, but inconsistently—some cylinders ignite properly while others misfire. Your engine runs on fewer cylinders than it should, which means more fuel gets burned in the catalytic converter instead of the combustion chamber.

That unburned fuel heats the catalytic converter to dangerous temperatures. Over time, the internal honeycomb structure breaks down and clogs. Once that happens, exhaust can't flow freely, backpressure builds, and your engine loses power. A clogged catalytic converter also triggers the check engine light and can't be cleaned—it has to be replaced, which costs $800 to $2,000 depending on your vehicle.

Bad spark plugs also damage the oxygen sensors that monitor exhaust composition. These sensors cost $150 to $400 each, and most vehicles have two. If multiple sensors fail because of prolonged misfiring, you're looking at $600 to $800 in sensor replacement alone, plus the catalytic converter work.

How far you can actually drive depends on driving conditions

Highway driving at steady speed is easier on a misfiring engine than city driving. On the highway, your engine runs at a consistent RPM and load, so the weak spark plugs can keep up most of the time. You might make 300 to 500 miles before the catalytic converter damage becomes severe enough to cause a noticeable loss of power or a stall.

City driving and delivery work are much harder on the engine. Constant acceleration, braking, and idling force the engine to work harder, which means the weak spark plugs fall further behind. Misfires become more frequent, more fuel flows to the catalytic converter, and damage accelerates. In heavy city traffic, you might only get 100 to 200 miles before the vehicle becomes difficult to drive or the check engine light comes on.

If you're doing food delivery, you're in the worst-case scenario: short trips, frequent stops, and constant acceleration. Bad spark plugs will make your vehicle sluggish, hurt your fuel economy, and increase the risk of a breakdown during a delivery. Replace them before you're stuck on the side of the road waiting for a tow truck.

Warning signs that spark plugs need replacement now

Rough idle is the most common sign. When you're stopped at a red light or parked, the engine shakes or vibrates more than normal. This happens because one or more cylinders are misfiring—the spark plug isn't firing consistently, so that cylinder doesn't contribute power on every stroke.

Hard starting means the engine cranks longer than usual before it catches. Bad spark plugs make ignition less reliable, especially when the engine is cold. If your vehicle takes 3 to 5 seconds to start instead of 1 to 2, spark plugs are usually the culprit.

Misfiring under acceleration feels like a hesitation or stumble when you press the gas pedal. The engine loses power momentarily, then catches up. This is a cylinder misfiring because the spark plug can't ignite the fuel reliably at higher RPM.

Poor fuel economy happens because misfires waste fuel. Your vehicle has to burn more gas to produce the same power, so you fill up more often. If your MPG drops 10 to 15% suddenly, have the spark plugs inspected.

Check engine light comes on when the engine computer detects misfires. The light alone doesn't tell you the problem is spark plugs—it could be a coil pack, fuel injector, or compression issue—but a misfire code almost always points to spark plugs first. Get the vehicle scanned at a shop to confirm.

How often spark plugs actually need replacement

Replacement intervals vary by spark plug type and your vehicle's design. Copper spark plugs last 25,000 to 30,000 miles and are the cheapest option. Iridium spark plugs last 60,000 to 100,000 miles and cost more upfront but save money over time. Platinum spark plugs fall in the middle at 30,000 to 50,000 miles.

Check your owner's manual for the exact interval your vehicle needs—it's usually printed in the maintenance schedule. If you've never replaced the spark plugs and you don't know when they were last done, assume they're due if your vehicle has over 30,000 miles. A shop can pull one out and measure the gap to see how worn they are, which takes 10 minutes and costs nothing.

If you drive a lot for delivery work, your spark plugs wear faster than the manufacturer's estimate because you're running the engine harder and more often. Consider replacing them at the lower end of the interval—every 25,000 to 30,000 miles instead of waiting for 100,000.

What to do if your check engine light is on

Don't ignore it, especially if you're using your vehicle for delivery. The light means the engine computer has detected a problem that could damage the catalytic converter or other expensive parts. Get the vehicle scanned at a repair shop or auto parts store—most places scan for free or charge $20 to $50.

The scan will show you a code like P0300 (random misfire) or P0301 (cylinder 1 misfire). These codes point to spark plugs, but they can also mean a bad coil pack, fuel injector, or low compression. A technician can narrow it down by checking the spark plug gap, testing the coils, and looking at fuel pressure. Don't drive long distances while the light is on—you risk catalytic converter damage that will cost far more than spark plug replacement.

If you're in the middle of a delivery shift when the light comes on, finish the current delivery and head to a shop. Don't push through multiple deliveries hoping it goes away. The longer you drive, the more damage accumulates, and you might end up stranded far from home.

The cost of waiting versus replacing spark plugs now

Spark plug replacement is one of the cheapest preventive maintenance items on your vehicle. A set of four spark plugs costs $50 to $200 depending on the type, and installation takes 30 minutes to 2 hours depending on your engine layout. Total cost: $100 to $400 at most shops.

If you wait until the catalytic converter is damaged, you're looking at $800 to $2,000 for replacement, plus $150 to $400 per oxygen sensor if those fail too. Engine damage from prolonged misfiring—like a cracked head or warped block—can cost $1,500 to $5,000 to repair. The math is clear: replace spark plugs on schedule and avoid the bigger repairs.

For delivery drivers, bad spark plugs also mean worse fuel economy, which cuts into your earnings. If your MPG drops from 25 to 20, you're spending an extra $20 to $30 per week on gas. Over a few months, that adds up to more than the cost of new spark plugs.

Frequently Asked Questions

Can bad spark plugs cause a complete engine shutdown?

Not usually. Bad spark plugs cause misfires and loss of power, but the engine will keep running on the cylinders that still fire. You'll lose performance and fuel economy, but you won't suddenly stall unless multiple plugs fail at once or the catalytic converter becomes so clogged that backpressure chokes the engine. That takes weeks or months of driving on bad plugs.

Will bad spark plugs damage my engine if I drive on the highway?

Highway driving is easier on a misfiring engine than city driving, but damage still happens. Unburned fuel flows to the catalytic converter and damages it over time. The longer you drive, the worse the damage becomes. You can make it several hundred miles, but you should replace the spark plugs as soon as you can, not wait until the next scheduled maintenance.

How do I know if it's spark plugs and not something else?

Get the vehicle scanned for diagnostic trouble codes. Misfire codes (P0300 series) usually point to spark plugs, but they can also mean bad coil packs or fuel injectors. A technician can pull a spark plug and check the gap and condition, which takes 10 minutes. If the plugs look worn or the gap is too wide, that's your answer.

Can I replace spark plugs myself to save money?

Yes, if you have basic tools and your engine layout allows straightforward access. Some vehicles have plugs that are straightforward to reach; others require removing the intake manifold or other components. Check a repair manual for your specific vehicle. If you're not comfortable doing it, a shop is worth the labor cost to make sure the job is done right.

What if I'm delivering and my car starts misfiring mid-shift?

Finish your current delivery and head to a shop or home, whichever is closer. Don't push through more deliveries—each mile increases the risk of catalytic converter damage. Call your delivery app and let them know you're taking the vehicle offline. It's better to lose a few deliveries than to damage your engine and lose the vehicle for days while it's being repaired.