Why is My Check Engine Light On? 9 Common Causes and What to Do

The dreaded check engine light. That glowing orange or yellow illuminated engine symbol that seems to be taunting you from your dashboard. As much as you try to ignore it, the check engine light just won't go away. We've all been there. But while it may be tempting to cover up the light and hope it disappears, the check engine light is trying to tell you something important. Ignoring it for too long can turn a minor issue into a major engine failure down the road.

So when that check engine light rears its ugly head, it's important to find out the root cause and take action. The light can come on for a number of reasons, ranging from a simple quick fix to a more complex repair. Read on to learn about the most common causes of a check engine light and what you should do when yours lights up.

1. Loose or Faulty Gas Cap

The most common cause for a check engine light is a loose or faulty gas cap. This accounts for nearly 15% of check engine light issues, according to When the gas cap is loose, cracked, or damaged, it allows fuel vapors to escape from the fuel tank. This trips the check engine light sensor. An easy fix is to tighten the gas cap or replace it completely if it’s damaged. This may instantly turn off the check engine light. If not, you may need to drive for a few days to allow the computer to reset and turn the light off.

2. Internal Engine Issue

An internal engine issue can also trigger the check engine light. This is a more serious cause and indicates a problem like low compression, failed gasket, sensor malfunction, or timing chain issues. The car’s computer detects abnormal readings from various sensors that monitor different engine functions. For example, the oxygen sensor monitors the air to fuel ratio and signals issues. Internal engine problems require a trip to your mechanic for proper diagnosis and repair.

3. Faulty Catalytic Converter

The catalytic converter plays a crucial emissions control role in your vehicle. It converts harmful exhaust from the engine into less toxic gases before releasing them into the atmosphere. When it starts to fail, it can’t properly clean the exhaust gases. This will switch on the check engine light. Replacing a faulty catalytic converter is expensive, often costing $1000 or more for parts and labor. Your mechanic may recommend an entire exhaust system replacement if the converter is damaged.

4. Bad Spark Plug Wires

Faulty spark plug wires are another common culprit for the dreaded check engine light. Spark plug wires channel the high voltage current that ignites the fuel and air mixture in the engine cylinders. When they are damaged or wearing out, the ignition system is disrupted. The check engine light provides an early warning of potential misfire issues. Replacing old spark plug wires with newer performance wires is a relatively easy and inexpensive repair.

5. Broken Gas Cap

As mentioned earlier, a faulty gas cap can cause the check engine light to come on. If the gas cap is cracked or the seal is broken, fuel vapors can leak out when the engine is running. The check engine light sensor will detect emissions escaping from an ineffective gas cap. Simply replacing the broken gas cap with a new one can instantly solve the problem. Be sure to tighten the new cap properly to prevent evaporation.

6. Faulty Oxygen Sensor

The oxygen sensor analyzes the amount of unburnt oxygen in the exhaust and feeds data to the car’s computer for emission control. When it starts to fail, the sensor provides inaccurate readings. This trips the check engine light due to the sensor indicating high oxygen levels from inefficient fuel combustion. Oxygen sensors eventually degrade over time and need replacement. Your mechanic should handle this repair since it involves removing the old sensor and installing a new one properly.

7. Low Oil Pressure

If you notice the check engine light along with low oil pressure warnings, it likely indicates your engine oil level is dangerously low. Running your engine without enough oil can cause severe internal damage. The light serves as an urgent signal to add oil immediately to prevent catastrophic failure. Determine if oil leaks are causing the low oil level. It may require repairing seals, gaskets or other components to prevent ongoing leaks after topping up the oil.

8. Overheating

Engine overheating is another critical cause of the check engine light. Modern vehicles have coolant temperature sensors that detect when the engine is running too hot. Overheating can happen due to low coolant, a faulty thermostat, busted radiator hose, or broken water pump. Driving a vehicle while it's overheating can warp cylinder heads and blow head gaskets. Stop driving immediately if your check engine light comes on along with temperature warning lights. Have the cooling system inspected and repaired before continuing to operate the overheated engine.

9. Fuel System Issues

Problems with the fuel system like clogged fuel filters, faulty fuel injectors, or failing fuel pump can also trigger the check engine light. Fuel system components send data signals to the computer about fuel pressure, volume, and efficiency. When they detect performance issues, it activates the warning light. Because fuel systems are complex, it’s best to see a professional mechanic to accurately diagnose and repair the problem.

What to Do When Your Check Engine Light Comes On

Now that you know some of the most common issues that cause the check engine light, here are tips on what to do when yours illuminates:

  • Immediately check your dashboard for any other warning lights to help identify the issue. Overheating or low oil pressure lights point to critical problems. Strange noises, smells or engine behavior may also accompany the check engine light.

  • Connect an OBD2 scanner tool to read and clear any diagnostic trouble codes stored in the computer. This can provide insight into what triggered the light. Auto parts stores will often scan codes for free.

  • Head to a professional mechanic for a computer diagnostic test. They have advanced scanners to hone in on the root cause and recommend repairs.

  • Don't simply reset the check engine light without addressing the underlying problem. This will only cause the light to come back on and doesn't fix the issue.

  • If it's a simple fix like tightening the gas cap, make the repair right away before the problem worsens.

  • For minor issues, you can disconnect the battery cables for a few minutes to temporarily reset the light. However, it will turn back on if the problem persists.

  • Don't ignore the light for an extended time. Driving with a known issue can lead to very expensive damage down the road.

  • If you recently had repairs done, the mechanic may not have reset the light. Ask them to scan for codes and reset the system.

  • Keep an eye out for any reduction in performance, mileage or other symptoms after the light appears. This helps the mechanic diagnose.

  • On older vehicles, disconnecting the battery may reset your radio or clock. Be prepared to reprogram them.

  • Some auto parts stores can lend out OBD2 code scanners for free to read check engine light codes.

  • When selling your vehicle, make sure to disclose any active check engine lights to avoid liability.

  • If the light is flashing or blinking, stop driving right away. This indicates a severe engine misfire.

  • After repairs, drive conservatively for a day before revving the engine to allow resetting the light.

  • Finding and fixing the issue quickly can prevent costly damage down the road and improve your vehicle's longevity.

Should You Keep Driving with the Check Engine Light On?

When that ominous glow appears, one of the first questions that pops into your mind may be: Is it safe to keep driving with the check engine light on? The answer depends on the cause and severity of the issue triggering the light.

For less serious problems like a loose gas cap, you can safely drive for a few days until you can make the repair. However, it's best to address even minor issues quickly to prevent them from escalating over time.

If the check engine light is accompanied by other warning lights or concerning symptoms, you should stop driving right away. For example, low oil pressure, overheating, or flashing check engine light require immediate attention to avoid destroying your engine.

Here are some guidelines on when it's okay to keep driving and when you need to stop right away:

Green Light to Keep Driving

  • No other symptoms or warning lights
  • Engine running smoothly
  • Normal temperature gauge reading
  • Recent repair work done
  • Gas cap loose or needs replacement

Red Light to Stop Driving

  • Overheating warning light/high temperature
  • Low oil pressure warning
  • Reduced engine power message
  • Flashing check engine light
  • Strange noises or smells
  • Visible smoke or leaks

Use your best judgment based on the situation. When in doubt, have your vehicle towed or serviced rather than risking thousands in repairs by continued driving. Addressing the issue early provides peace of mind that you're not causing catastrophic damage.

How Long Can You Drive with Check Engine Light On?

Many people wonder how long they can safely drive once the check engine light pops up. As a general rule, the sooner you can diagnose and repair the problem, the better. Driving for long periods with a known issue that causes the warning light risks further damaging related components.

However, if your vehicle seems to be running fine otherwise, here are general guidelines for maximum recommended driving distances with the check engine light illuminated:

  • Gas cap issue - 200 miles
  • Oxygen sensor problem - 500 miles
  • Spark plug or wire issue - 500 miles
  • Catalytic converter problem - 150 miles
  • Engine misfire - 50 miles
  • Flashing check engine light - Do not drive, tow vehicle

Keep in mind these recommendations assume no other performance symptoms or warning lights beyond just the steady check engine light. Extensive driving with a catalytic converter, oxygen sensor, or misfire problem can lead to very pricey repairs. Check with your mechanic for tailored advice based on your specific situation.

Top Causes of Intermittent Check Engine Light

In some cases, the check engine light doesn't stay on, but rather flickers on and off intermittently. This can happen with electrical issues that prevent the light from illuminating until the connection is restored. Intermittent check engine lights point to wiring harness, computer, or sensor problems.

Here are some typical causes of an intermittent check engine light:

  • Loose wiring harness connection
  • Faulty computer or ECU
  • Corroded or loose wiring
  • Weak ignition coil
  • Dirty mass airflow sensor
  • Bad crankshaft sensor
  • Vacuum leaks
  • Defective thermostat

Diagnosing an intermittent light can be tricky. The mechanic may need to take your car for a road test when the light is on to properly scan diagnostic codes. Reliably replicating the issue is key.

Keep notes on exactly when the intermittent light illuminates during your driving. This helps the mechanic narrow down the electrical or sensor fault. Addressing an intermittent light prevents getting stranded when the light finally stays on for good.

The Bottom Line on Check Engine Lights

While it can be alarming seeing your check engine light come on, it is designed to alert you early on about problems under the hood. This provides an opportunity to address small issues before they snowball into major repair bills.

Use the warning light as a chance to be proactive about your vehicle's health. Diagnose the cause, make necessary repairs promptly, and take action at the first sign of trouble to avoid breakdowns and expensive damage. With proper care when the check engine light appears, your car can keep running smoothly for many more miles down the road.