You might be feeling your car running rough, shaking when you start it, or maybe the engine light is on. You’re wondering how long can you drive with bad spark plugs without ruining your car. The quick answer is—you shouldn’t drive more than a few days with bad spark plugs. Pushing it longer can cause engine damage, poor fuel economy, and leave you stranded. In this article, we’ll walk you through the full truth, the risks, the signs, and what happens the longer you wait to fix them.
Your Engine Feels Weak or Jumpy
When spark plugs go bad, your car doesn’t run smooth anymore. It may shake a bit when starting or while driving. You might feel it jerking, like the power is cutting in and out. That’s because spark plugs help light the fuel in your engine. If they don’t work right, your engine doesn’t burn fuel the right way.
You might still be able to drive, but it won’t be safe for long. The more you drive, the worse it can get. Your car may misfire more, lose power, and use more fuel. Sometimes, you won’t be able to go up hills well, or your car may stall in the middle of traffic. These little things can become big problems if you ignore them.
Most people can still drive 50 to 100 miles when the plugs start to fail, but it’s not a safe bet. Some people go longer, but that’s risky. It’s like walking on ice—you never know when you’ll slip. If your spark plugs are bad, fix them soon. Waiting too long can damage your engine or even your catalytic converter, which is costly to replace.
- Car shakes when starting or driving
- Power feels weak or jumpy
- Risk of car stalling in traffic
- Bad fuel burn leads to poor mileage
- May damage engine if ignored
- Shouldn’t drive more than a few days
Gas Mileage Drops and Fuel Gets Wasted
One clear sign of bad spark plugs is when your car starts using more gas than before. You’ll notice that your fuel tank empties quicker, even if you drive the same distance. This happens because the engine is working harder but not burning fuel well. It’s like cooking with a weak flame—it takes longer and wastes energy.
When the spark plug doesn’t fire correctly, the engine tries to make up for it. This uses more fuel than needed. You may even start smelling gas near your car’s exhaust. That’s unburnt fuel going out. This doesn’t just waste money, it pollutes the air and damages your car parts too.
Fuel economy can drop by 20% or more. That means if your car gave you 25 miles per gallon before, it may now give only 18–20. That’s a big difference over time, especially if you drive a lot. You’ll spend more money on gas every week, and it adds up fast.
A car running like this also puts extra stress on your engine. It’s like jogging with a heavy backpack. The engine gets tired faster and wears out sooner. So not only are you losing fuel, but you’re also damaging your engine in the process.
- Fuel runs out faster
- Gas mileage drops by 20% or more
- Unburnt fuel smells in the air
- Extra engine strain from wasted fuel
- More frequent fuel stops
- Car costs more to run each day
Engine Misfires Get Worse Over Time
When a spark plug is just starting to fail, you may not notice a big issue right away. But the problem grows. One day your car skips a beat, then a few days later it happens more often. These are engine misfires. It feels like the engine coughs or jerks while driving.
A misfire happens when the fuel in one of your engine cylinders doesn’t burn at the right time. This can cause loud sounds from the engine, shaking, or a loss of power. If this keeps happening, it damages your engine parts over time. The pistons, valves, and other parts take extra stress, which can cost a lot to repair.
Sometimes, a misfire can be strong enough to stop your engine. That can be scary if you’re in the middle of the road. Also, misfires dump raw fuel into your exhaust, which can damage your catalytic converter. That part alone can cost $1,000 or more to fix.
The longer you drive with bad spark plugs, the more frequent the misfires become. This turns from a small fix to a big engine repair. What could have been a $50 spark plug change may turn into a $2,000 repair job.
- Misfires grow worse day by day
- Engine loses timing and power
- Pistons and valves get damaged
- Risk of full engine breakdown
- Can cause loud noise and shaking
- Expensive repairs if ignored too long
Your Car May Refuse to Start
If the spark plugs are in really bad shape, your car might not start at all. You turn the key or press the button, and nothing happens. Or maybe it cranks for a long time but doesn’t catch. That’s because the engine can’t get the spark it needs to start.
Spark plugs are what ignite the fuel-air mix in the engine. No spark, no fire, no start. Sometimes this happens on cold mornings, when the battery and engine already have to work harder. Old or fouled spark plugs just can’t do the job.
This can leave you stranded in your driveway or worse—somewhere far from help. You’ll need a tow truck and a repair shop visit, both of which cost money and time. This kind of surprise is never fun and usually happens at the worst time.
You might think your battery is dead, but changing the battery won’t fix spark plugs. If your plugs are worn out, they need to be replaced. There’s no shortcut. Once the car refuses to start, it’s already gone too far.
- Engine won’t start without spark
- Long cranking but no ignition
- Risk of getting stranded
- Tow truck and shop visit needed
- Battery won’t fix the issue
- Plugs must be replaced to start the engine
Your Check Engine Light Stays On
That little yellow or orange light on your dash is trying to tell you something. When spark plugs go bad, your check engine light might come on. Many people ignore it, thinking it’s no big deal. But it’s actually your car’s way of shouting for help.
The engine control unit (ECU) tracks how your engine runs. When something isn’t right—like a misfire or poor fuel burn—it sends out a warning. Bad spark plugs are a common reason for this light to come on.
If you go to an auto parts store, they can scan your car for free. The scan may show codes like P0300 (random misfire) or P0301 to P0304 (cylinder misfires). These often point to spark plug issues.
Ignoring the light doesn’t make it go away. It gets worse. The car may go into “limp mode,” where it won’t drive faster than 30 mph. This is to protect your engine from damage. But it also makes the car hard to drive and unsafe in traffic.
- Check engine light is a clear warning
- Shows misfire codes when scanned
- Car may enter limp mode
- Warning won’t go away on its own
- Signals deeper engine trouble
- Needs fixing before it gets worse
Other Parts Can Get Damaged
Bad spark plugs don’t just stay a spark plug problem. They spread damage to other parts. The longer you drive with them, the more your car suffers.
The ignition coils can burn out because they have to work harder to create a spark. These coils are expensive and hard to replace. The fuel injectors can also get dirty or stuck, trying to make up for the spark problem.
Your catalytic converter is at big risk too. That’s the part that cleans your car’s exhaust. Unburned fuel from misfires gets into it, which can clog or melt it. Replacing this part costs more than most people want to spend.
Even your engine oil can get dirty faster when the fuel doesn’t burn cleanly. This makes your engine wear out quicker, reducing its life.
Driving with bad spark plugs is like ignoring a toothache. What starts small turns into a deep problem that needs surgery. Fixing plugs early saves time, money, and a whole lot of stress.
- Ignition coils can burn out
- Fuel injectors get dirty or stuck
- Catalytic converter may clog or melt
- Engine oil becomes dirty quicker
- Whole engine life is shortened
- Repairs multiply over time
Final Thoughts
Driving with bad spark plugs may not seem serious at first, but it leads to big problems fast. The longer you wait, the worse your car gets, and the more it costs. If your car shakes, misfires, burns more fuel, or won’t start easily, it’s time to check the spark plugs. Don’t take the risk of getting stuck or damaging your engine. It’s a small fix that can save you from big headaches down the road.
| Problem Sign | What Happens | Risk Level | Repair Needed |
|---|---|---|---|
| Engine shaking or weak | Poor performance and fuel burn | High | Replace spark plugs |
| Low fuel mileage | More gas used, costs more money | Medium | Replace spark plugs |
| Engine misfires | Loud noise, poor drive, engine harm | Very High | Spark plugs + inspection |
| Car won’t start | No ignition, car stuck | Critical | New spark plugs + test |
| Check engine light | Misfire code, ECU warning | High | Spark plug change needed |
| More parts get damaged | Coils, injectors, converter wear out | Extreme | Multi-part replacement |
Frequently Asked Questions (FAQs)
Is it safe to keep driving with bad spark plugs?
No, it is not safe to keep driving with bad spark plugs. Even if the car still runs, it can quickly turn dangerous. Bad spark plugs cause your engine to misfire, run roughly, and even stall. This makes it harder to control your car and can leave you stuck on the road. Also, the longer you drive, the more parts like ignition coils or the catalytic converter can get damaged. It’s always better to fix spark plugs as soon as the signs start showing.
Can bad spark plugs ruin my engine?
Yes, bad spark plugs can damage your engine if you drive too long with them. Misfiring causes your engine to run hot and rough. Over time, this can damage pistons, valves, and even the cylinder walls. If raw fuel enters the exhaust system, it can destroy your catalytic converter too. These repairs are very expensive and sometimes lead to full engine replacement. Fixing spark plugs early avoids all this.
Do I need to replace all spark plugs at once?
Yes, it’s a good idea to replace all the spark plugs at once. Most engines fire all plugs together, and if one has gone bad, the others are likely close behind. Replacing them all keeps your engine balanced and running smoothly. Also, changing just one doesn’t save much money and might lead to uneven performance. Full replacement gives you better fuel efficiency and fewer misfires.
Is it hard to change spark plugs by myself?
It depends on your car and your comfort level. Some cars make it easy, where you just need a spark plug socket and a little time. Others hide the plugs under engine covers or near the firewall, making it tricky. If you’re a beginner, watch some videos for your specific car. If it looks too tight or risky, getting help from a mechanic is smart. You don’t want to strip threads or break a plug inside the engine.
Can bad spark plugs affect acceleration?
Yes, they can. Bad spark plugs cause the engine to lose power or skip while accelerating. This makes your car feel slow, heavy, or jumpy. It also takes longer to speed up, especially when going uphill or merging onto highways. Good spark plugs give clean, strong sparks that help the engine respond fast when you press the gas. So if your car feels sluggish, the spark plugs could be the reason.
Is it normal for my check engine light to come on for spark plugs?
Yes, it’s normal. The check engine light turns on when your car’s computer sees something wrong—like a misfire or poor spark. If a spark plug fails, it often triggers a code like P0300 or P0301. These codes mean your engine isn’t firing right. It’s important to read the code and not ignore the light. Fixing the plugs usually makes the light go off.
Do I have to go to a shop to fix spark plugs?
Not always. If you have the tools and time, you can fix spark plugs at home. Many people change them in their driveway. You need a socket wrench, a spark plug socket, and sometimes an extension bar. Be sure you know the right plug gap and torque level. But if you’re not sure or can’t reach the plugs, a repair shop can help and do it fast.
Can bad spark plugs make my car shake?
Yes, they can. When spark plugs misfire, the engine becomes unbalanced. You’ll feel the shaking in the steering wheel, seat, or even the whole car. This gets worse when idling or at low speeds. It’s your engine struggling to run on all cylinders. Replacing spark plugs usually solves this shaking problem quickly, giving you a smooth ride again.