Both lights mean your engine computer has detected a problem it wants you to know about

When your check engine light and traction control light turn on at the same time, they are usually connected to the same underlying issue — not two separate problems. Your engine's computer (called the powertrain control module, or PCM) monitors dozens of sensors throughout your vehicle. When one of those sensors reports a reading outside normal range, the computer logs a fault code and illuminates both lights as a warning.

The check engine light is your vehicle's general alert that something in the engine, emissions system, or transmission needs attention. The traction control light turns on when the computer detects wheel slip or a problem with the system that prevents skidding. When they light up together, the computer is usually telling you that a sensor malfunction or engine misfire is affecting both systems at once.

You can drive the vehicle to a repair shop, but you should not ignore the lights or assume they will go away on their own. The problem will not fix itself, and continuing to drive may cause additional damage or reduce fuel economy.

Key Takeaways

  • A check engine light and traction control light appearing together usually point to a single fault code, not two separate issues.
  • Common causes include a faulty oxygen sensor, mass airflow sensor, spark plug problems, or a loose gas cap.
  • You will need a diagnostic scanner to read the fault code and identify the exact problem.
  • Some causes (like a loose gas cap) you can fix yourself; most require a mechanic or dealership.

The most common causes of both lights turning on

An oxygen sensor malfunction is the most frequent reason both lights appear together. Your vehicle has at least two oxygen sensors (one before the catalytic converter and one after). These sensors measure how much unburned oxygen is in the exhaust. When one fails, the engine computer cannot adjust the fuel mixture correctly, which triggers both the check engine light and traction control light because the engine is not running smoothly.

A mass airflow (MAF) sensor problem is the second most common cause. This sensor tells the computer how much air is entering the engine so it can adjust fuel delivery. A dirty or failing MAF sensor sends incorrect readings, causing rough idle, hesitation, and both warning lights.

A loose or missing gas cap can trigger both lights, though this is less common. The cap seals your fuel system. If it is loose or cracked, fuel vapors escape, the computer detects the leak, and both lights may illuminate. This is the easiest problem to check yourself — straightforward tighten the cap and see if the lights turn off after a few driving cycles.

Spark plug issues, a faulty catalytic converter, or transmission sensor problems can also cause both lights to appear. The only way to know which problem you actually have is to read the fault code from your computer.

How to read the fault code yourself

Your vehicle's computer stores a specific code that describes what went wrong. You can read this code using an OBD-II scanner (On-Board Diagnostic scanner), which plugs into a port under your steering wheel. Most auto parts stores offer free scanning — you drive in, they plug in the scanner, and they tell you the code and what it means. This takes about five minutes and costs nothing.

If you own a scanner or want to buy one, basic models cost $25 to $60 and work on most vehicles made after 1996. Plug the scanner into the diagnostic port (located below the steering wheel on the driver's side), turn on the ignition without starting the engine, and the scanner will display the fault code. Write down the exact code — it will be something like "P0135" or "P0171" — and search for that code online to understand what sensor or system is failing.

Once you know the code, you have a clear picture of what needs repair. Some codes point to problems you can fix yourself (like replacing a spark plug). Most point to sensor replacements or repairs that require a mechanic.

When you can fix it yourself versus when to see a mechanic

A loose gas cap is the only common problem you should attempt to fix yourself. Tighten it firmly, then drive the vehicle for 50 to 100 miles. If the lights were caused only by the loose cap, they should turn off after the computer completes a full diagnostic cycle. If the lights remain on, the problem is something else.

Spark plug replacement is possible for a DIYer with basic tools and mechanical confidence, but it varies by vehicle. Some engines have spark plugs that are straightforward to reach; others require removing the intake manifold or other components. If you are unsure, a mechanic can replace spark plugs in 30 minutes to an hour for $100 to $300 depending on your vehicle.

Sensor replacements (oxygen sensors, MAF sensors, catalytic converters) should go to a mechanic or dealership. These require specialized tools, proper installation technique, and often a retest to confirm the fault code is cleared. Dealerships typically charge $150 to $400 for sensor replacement plus diagnostics. Independent mechanics often charge less. Get a quote before authorizing work.

What happens if you ignore the lights

Ignoring both lights will not cause your vehicle to suddenly stop running, but it will cause problems over time. An oxygen sensor failure reduces fuel economy noticeably — you may see a 10 to 20 percent drop in miles per gallon. A failing catalytic converter can eventually clog, which restricts exhaust flow and causes the engine to overheat or lose power.

In some states, a vehicle with an illuminated check engine light will fail an emissions inspection and cannot be registered. If you plan to sell the vehicle, a dealer or private buyer will have the vehicle scanned before purchase, discover the fault code, and use it to negotiate a lower price or walk away from the deal.

The longer you wait, the more likely a single sensor failure will cause secondary damage. For example, a faulty oxygen sensor can damage the catalytic converter over weeks or months, turning a $200 repair into a $1,000 repair.

Steps to take right now

First, check your gas cap. If it is loose, tight it and drive for a few days to see if the lights turn off. This takes one minute and solves the problem in roughly 5 percent of cases.

Second, visit an auto parts store and ask them to scan the vehicle for free. Write down the fault code they give you. This tells you exactly what the computer detected and whether the problem is a sensor, spark plug, or something else.

Third, call a local mechanic or dealership and describe the fault code. Ask for a quote to diagnose and repair the problem. Most shops can give you a ballpark estimate over the phone once they know the code. Compare quotes from two or three shops before deciding where to take the vehicle.

Fourth, schedule the repair. Most sensor replacements take one to two hours. You can usually drop the vehicle off in the morning and pick it up the same day.

How to prevent this from happening again

Regular maintenance is the best prevention. Have your vehicle serviced according to the manufacturer's schedule, which usually means an oil change every 5,000 to 10,000 miles and a full inspection annually. During these services, a mechanic will check sensor condition and catch problems before they trigger warning lights.

Keep your gas cap tight and in good condition. If it cracks or the seal wears out, replace it — a new cap costs $10 to $20 and prevents fuel system leaks that trigger both lights.

Avoid driving through deep water or puddles that could damage sensors or electrical connectors. Sensors are sealed, but water can still find its way in if you drive through standing water regularly.

Use quality fuel and change your engine air filter every 12,000 to 15,000 miles. A clogged air filter forces the engine to run rich (too much fuel, not enough air), which can trigger both lights and reduce performance.

Frequently Asked Questions

Can I drive the car if both lights are on?

Yes, you can drive to a repair shop, but avoid highway speeds or heavy acceleration. The vehicle is running in a reduced-power mode to protect the engine. Get it scanned and repaired within a few days to prevent additional damage.

Will the lights turn off on their own?

No. The lights will remain on until the underlying problem is fixed and the fault code is cleared from the computer's memory. Even if you clear the code with a scanner, the lights will return if the problem still exists.

How much will this repair cost?

It depends on the fault code. A loose gas cap costs nothing. Spark plug replacement runs $100 to $300. Sensor replacement (oxygen, MAF, or catalytic converter) ranges from $150 to $800 depending on which sensor and your vehicle type. Get a quote from your mechanic after they read the code.

Should I go to a dealership or an independent mechanic?

Independent mechanics are usually cheaper and equally capable for sensor and spark plug work. Dealerships are useful if your vehicle is under warranty or if the problem is complex. Call both and compare quotes before deciding.

What if the code comes back after I fix it?

If the same fault code returns after repair, the replacement part may be defective, or the problem may be elsewhere in the system (like a wiring issue or a second failing sensor). Return to the shop that did the work — most offer a warranty on parts and labor for 30 days.