Why Is My Check Engine Light On?

Why Is My Check Engine Light On?
Yaroslau Mikheyeu/Shutterstock

You’re driving down the interstate and all is well. Then, ding. A little yellow light shines on your dashboard. That malfunction indicator lamp crudely shaped like an engine causes your heart to sink. Is the motor going to fall out of the chassis at 70 miles per hour? In all likelihood, it won’t, but there are reasons for check engine light illumination.

A warning indicator is among the most common problems you can experience in any vehicle. What are the reasons for the check engine light, and what do you need to do to get rid of it? Read on to find out.

Looking at a used car? See if there’s anything in its history you’d want to know about beforehand with a VIN Lookup – run 50 vehicle searches per month with a Bumper subscription!

11 reasons the check engine light comes on

The check engine light is part of a vehicle’s onboard diagnostics system. Contrary to its name, it doesn’t actively monitor the engine at all. Some engine-related systems are included, like the fuel system and air intake, but for the most part it’s about emissions issues—sensors and valves and the like. Major mechanical parts don’t trigger the check engine light in the vast majority of situations. If yours lights up, these 11 systems are the most likely culprits.

1. Oxygen sensor

An oxygen sensor, or O2 sensor, monitors the amount of oxygen in a vehicle’s exhaust. An element mounted in the exhaust pipe produces voltage from exhaust oxygen levels, and the voltage is indexed to a value in the onboard diagnostic system. If the value is either higher or lower than the reference, it triggers a DTC, or diagnostic trouble code. These codes may be one of the reasons the check engine light comes on.

Most cars have two or more oxygen sensors. Upstream O2 sensors or precatalytic converter oxygen sensors regulate the air-fuel mixture, while downstream O2 sensors monitor the catalytic converter’s efficiency.

An oxygen sensor is an inexpensive piece and simple to change. An average replacement cost is typically between $300 and $400, including parts and labor.

2. Fuel cap

This seemingly unimportant part is probably the top reason people ask, “Why is my check engine light on?” Everyone knows what a fuel cap is: The twist-on plastic cap covering the fuel filler neck. While it prevents dirt and moisture from entering the fuel system, it performs another role almost no one knows, allowing the fuel system to pressurize.

Cars perform a self-check for fuel system leaks in the background. When the system tries to pressurize while the fuel cap is loose, missing or broken, the computer sees a massive leak, triggering the check engine light.

This is among the top check engine light causes and the cheapest to fix. Usually it’s just a matter of tightening the cap, but even if the fuel cap needs to be replaced, the cost averages around $80.

3. Catalytic converter

When gasoline burns in the engine, the chemical reaction creates compounds like nitrous oxides and hydrocarbons that enter the atmosphere. Because these harmful chemicals contribute to smog and poor air quality, catalytic converters were implemented in the late 1960s. Shaped like a muffler, the converter’s interior has a ceramic, honeycomb-like structure covered in reactive elements such as platinum, which triggers a response when exposed to the aforementioned chemicals. The converter superheats the compounds and breaks them down into harmless molecules, such as carbon dioxide and water, that exit the tailpipe.

Low-quality fuel and worn engines that burn oil tend to contaminate the interior of a catalytic converter, causing it to overheat and break apart. When the catalytic converter isn’t operating efficiently anymore, a downstream oxygen sensor detects a change and triggers the check engine light.

As one of the more expensive repairs related to the check engine light, replacing a faulty catalytic converter might make you fidget. Expect a bill around $1,850.

4. Mass air flow sensor

For an efficient burn, the engine’s air-fuel mixture needs to be perfect. Not only does the amount of fuel need to be metered, but the air taken in also needs to be calculated accurately. The mass air flow sensor performs that role.

Positioned in the air intake duct, the sensor’s element can get dirty. The connector is relatively exposed under the hood, so a slight impact could crack it or damage a wire. If the mass air flow sensor can’t accurately read the engine’s air volume, you could risk engine damage by running lean (not burning as much fuel as it should) or rich (burning excess fuel), so it triggers a check engine light.

Prices vary for a mass air flow sensor replacement. Expect somewhere between $292 and $407, or an average around $350.

5. Spark plugs

Spark plugs ignite the air-fuel mixture that enters the combustion chamber. Typically screwed into the intake manifold with the electrodes exposed inside the engine, a tiny but powerful spark starts the controlled explosion that produces engine power.

Several things can go wrong with spark plugs. Over time, the electrodes can wear down or the porcelain insulator can crack. They can be fouled by a flooded engine or oil contamination. They can also overheat if the engine is running too lean. If a spark plug fails to ignite on a consistent basis, it triggers a misfire code in the onboard diagnostic system.

Changing one faulty spark plug is around $10 to $25, depending on the engine and its requirements. It’s usually best to change them as a set because the rest are usually in similar condition.

6. Ignition coil

The ignition coil delivers the electrical pulse to each spark plug, causing them to spark. The ignition coil has a coil of wire inside that magnifies a small electric charge into a much larger one and stores it momentarily. When the engine computer sends a signal, the coil releases the pent-up energy to the spark plug, where it ignites the air-fuel mixture. Most cars have one coil per spark plug.

Ignition coils are prone to failure after several years. If the rubber boot breaks down, it can arc to other metal parts and degrade the charge delivered to the spark plug. Or it might not fire at all. Problems include poor fuel economy and decreased engine power, and the check engine light can come on for a misfire code.

It’s a straightforward repair to swap out an ignition coil for most cars. On average, it’s between $201 and $269 per ignition coil, with most of the cost being parts.

7. Fuel injector

Whether you have a gas-powered car or a vehicle with a diesel engine, each cylinder has a fuel injector. It’s a small, electronically-activated valve that regulates how much fuel is sprayed into the cylinder during the intake cycle.

The fuel your engine burns, gas or diesel, tends to have impurities. Those impurities, as well as carbon from the combustion process, can cause the minuscule holes in the injector tip to plug or clog altogether. Occasionally, a fuel injector can be completely clogged, and an injector can stick open and continuously leak fuel into a cylinder.

A faulty fuel injector can cause the engine to run rough because of a misfire either from too little or too much fuel. This might only show up when you’re heavy on the throttle as well.

Fuel injectors are very precise metering devices, and they’re costly as a result. To replace a fuel injector averages around $720 per injector.

8. Vacuum leak

Car systems like the throttle, emissions and power brakes use engine vacuum pressure to assist them. Hoses and valves draw a vacuum from the engine’s intake manifold.

Should one of these hoses break or be dislodged, or if a valve gets stuck open, the onboard diagnostics usually see a leak. You might notice the engine idling faster, running rough, stalling or making a whistling sound, and it’s a safe bet the check engine light comes on.

Repairing the problem could be as simple as reconnecting a hose—or as expensive as replacing a cracked intake manifold for $1,000 or more. It really depends on the root cause.

9. Ignition wires

Some cars deliver the spark from the ignition coil to the spark plug through an insulated cable. Ignition cables, also known as ignition wires or spark plug wires, have a boot on either end to insulate the metal clip from arcing. Wrapped in rubber, the ignition wires are designed to deliver the spark to the spark plug without degradation.

Potential problems come along with any kind of damage to a spark plug wire. Cracked wire insulation can cause arcing, usually accompanied by visible burns or discoloration on the spark plug wire. A pinch or impact can sever the wire inside. Any damage causes a misfire because the spark plug can’t ignite.

Few jobs are as easy as changing out spark plug wires. To replace the whole set, expect to pay between $184 and $222 for parts and labor.

10. Evaporative emissions purge control valve

In the emissions control system, fuel can evaporate, but vapors escaping into the atmosphere can be harmful. An evaporative emissions system canister filters fuel vapors and condenses them back into fuel, but some vapors remain. The evaporative purge control valve releases those vapors into the combustion chamber to be burnt off rather than letting them go into the air.

An evaporative purge control valve can become plugged, stick open or get stuck shut. The connector can also corrode and lose communication with the valve. When that happens, the onboard computer triggers an emissions-related DTC and the check engine light comes on.

Between parts and labor, the average purge valve is between $110 and $170.

11. Engine thermostat

A car’s engine runs best between 195 and 220º F. Lower temperatures require more fuel to run properly while higher temperatures create more harmful emissions. The engine thermostat regulates the coolant temperature. The thermostat contains a wax pellet that controls when it opens and closes. As the temperature reaches the right zone, the thermostat opens to allow coolant to flow to the radiator. The thermostat closes when it cools below that comfy window.

Thermostats are simple devices but they’re likely to fail at some point. Poor maintenance can mean calcified deposits get stuck in the thermostat, keeping it open so the engine doesn’t achieve the best temperature. The thermostat can also close, causing high temperatures or overheating. Both are reasons the check engine light come on.

Changing out a thermostat on most cars is a routine job for pros and DIYers alike. Average costs are around $231 for parts and labor if you have a shop do the work.

How to tell what could cause the check engine light to come on

The onboard diagnostics system has an access port under the dashboard. This is the connector you’ll tap into whenever it’s necessary to delve into the vehicle’s computers to diagnose a problem. It’s a 16-pin connector shaped like an inverted trapezoid.

The DTCs stored in the control modules need to be deciphered to discover what the problem is. A code reader is what you’ll require to do that. Until the widespread implementation of OBD-II technology in the mid-1990s, code readers were mostly make-specific and usually required a specialized tech to scan for codes. With OBD-II and newer standards, most scan tools or code readers can find the fault codes or DTCs for almost any make and model—at least for check engine light causes.

Reading trouble codes requires an OBD-II compatible scan tool. Plug it into the OBD connector under the dash and, after following the prompts on the device, read the DTCs. Each code is five digits, beginning with one letter and followed by four numbers. The exception is for manufacturer-specific codes that may include additional letters in the trouble code.

For fault codes common to all models, the code begins with P followed by a zero. After that, the next number is specific to the subsystem where the fault resides:

  • P00xx indicates either fuel and air metering or auxiliary emission controls
  • P01xx indicates fuel and air metering systems
  • P02xx points to injector circuit faults in the fuel and air metering system
  • P03xx signals an ignition system fault or a misfire
  • P04xx is less common as auxiliary emissions controls codes
  • P05xx indicates vehicle speed controls and idle control system failures
  • P06xx isn’t seen very often in the computer output circuit subsystem
  • P07xx codes are related to the transmission
  • P08xx codes are also transmission-related

Code readers range from beginner models that have two buttons and smartphone-controlled Bluetooth scan tools to novice and professional-grade devices that scan generic and manufacturer-specific codes. Basic units will likely only provide diagnostic codes for check engine lights, while more advanced tools could scan for antilock braking system and airbag system codes and program keys and perform high-level systems tests.

You might be able to find a cheap Amazon or parts store code reader for as little as $20 if all you need to do is read DTCs and clear codes. If you want freeze frame data, systems monitoring or any other advanced information, scanners can be thousands of dollars.

Can I use the code to fix the problem myself?

Think of a DTC like a scavenger hunt. Once you have a clue, you can begin to solve the problem. Codes are like that—they don’t bring you right to the solution in many cases, but they point you in the direction where you should search for a problem.

It’s rarely as simple as changing out a part. For example, an O2 sensor code could be a problem with poor fuel quality, an exhaust system leak or a misfiring cylinder. Or it could be a faulty O2 sensor in the end. But you need to rely on mechanical knowledge and diagnostic skills beyond just DTCs.

How to turn off the check engine light

Let’s say you’re sure you’ve fixed all the reasons for check engine light illumination in your car. Maybe it was a loose gas cap and you’ve tightened it, or the mass air flow sensor was obviously damaged and you’ve replaced it. You’re asking yourself, “Why is my engine light on?” Those DTCs don’t always clear themselves, and when they do, it’s usually after a handful of successful self-test sequences taking several drives. Manually clearing the check engine light after a repair is the better option.

To turn off the light, you can try a few things:

  • Disconnect both the battery cables from the battery and touch the positive and negative cable ends. Hold them together for five minutes to discharge any residual charge in the system. It’s not an advised method, but it can sometimes work in a pinch.
  • Pull the fuse for the engine control module and re-install it. For newer models, this is unlikely to work, but there’s a possibility it could work in older cars.
  • Clear the DTCs with a code reader. With almost any code reader, you can either read the codes and clear them, or skip right to clearing codes. It’s also a great way to start diagnosing a problem when there are multiple DTCs in the system. If any codes return, you know where to start with your diagnosis.

About Bumper

At Bumper, we are on a mission to bring vehicle history reports and ownership up to speed with modern times. A vehicle is one of the most expensive purchases you'll likely make, and you deserve to have access to the same tools and information the pros use to make the right decisions.

About Jason Unrau

Jason Unrau is an expert automotive writer with more than 21 years of auto industry experience, first in auto dealerships for 15 years and then as a writer. Having grown up around cars, the feel of a wrench became familiar for him and before graduating from high school, he had rebuilt engines and carburetors on personal vehicles. After school, Jason entered the workforce at a car dealership and worked his way through several positions in both sales and service. Jason has in-depth knowledge of the automotive industry at the dealership level along with repair information. Now, as a full-time writer, he writes engaging content in all different aspects of the automotive industry.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.