How to Diagnose a Flashing Check Engine Light and Shaking Car

• The flashing check engine light and shaking both point towards an engine misfire.
• The problem is likely a bad spark plug or ignition coil, so replacing these engine components should fix the problem.
• Do not continue to drive with a flashing check engine light and a shaking car. Pull over to the side of the road and call a tow truck.

What causes a flashing check engine light and shaking?

Your engine is almost certainly misfiring. The flashing check engine light means that there’s a serious problem with your engine’s functioning, and almost always indicates your engine is misfiring for some reason. The shaking also points towards a misfire, since a vehicle will shake during a misfire. That rattling will increase the faster you’re going. To be specific, a misfire occurs when one or more of the cylinders in your engine isn’t moving, or when it moves at a rate it shouldn’t be firing. Misfires will often mess with your acceleration. You may feel like your vehicle is losing power or you have to press the gas pedal harder to get up to speed. A solid check engine light indicates that there’s an error detected in the engine or emissions system. This is also a big deal, but it’s less immediately serious than a flashing check engine light.

Common Underlying Causes

Bad spark plugs This is the #1 culprit of these symptoms (especially on older vehicles). The spark plugs provide the spark needed to ignite fuel, and it’s one of the three ingredients needed in the fuel + air + spark formula that your engine needs to fire properly. Without spark in one or more of the cylinders, that fuel and air won’t ignite and your engine will misfire. This will cause shaking at idle that gets worse when you accelerate. The Fix: Replace your spark plugs. Remove your fuel rail to access your spark plugs yourself, or take your vehicle to a mechanic. Change all of your spark plugs at one time. If one of them has gone bad, the odds that others are about to fail are especially high. You have one spark plug for each cylinder. So, if you have a V6 engine, you have 6 spark plugs. You need to change your spark plugs every 100,000 miles (160,000 km) or so.

Faulty ignition coils If your spark plugs happen to be fine, the most likely problem is your ignition coils. The spark plugs send their spark through these coils to reach the cylinder, and the coil carries that current and distributes it smoothly. Without a functioning coil, the cylinder either won’t receive the spark, or the spark will be unevenly distributed and the cylinder will misfire. This will cause symptoms identical to bad spark plugs. The Fix: Replace your ignition coils. This repair is a little more intricate than swapping out the spark plugs, so see a professional mechanic if you can.

Airflow sensor issues Is the shaking you experience when you accelerate more of a jerking forward or stuttering? Does your car feel too fast, then too slow? If so, the problem is likely your airflow sensor. A bad airflow sensor will cause an inconsistent amount of air to flow into the engine, which can lead to misfires. The Fix: Your sensor is most likely dirty, so read your manual to locate it, and unscrew it from your engine. Clean the sensor with any MAF cleaner and reinstall it. If cleaning the sensor doesn’t solve the problem, it needs to be replaced. Purchase a model designed for your vehicle and install it. Some people will try to bypass a bad air flow sensor to keep it from triggering problems. Unfortunately, this will eventually lead to serious engine damage. Don’t do it.

Less Common Causes

Air mixture issues Any interruption to the airflow that your engine uses in the fuel + air + spark formula will also cause your engine to misfire. The most common culprits here are a vacuum leak, which will cause tons of extra air to enter the engine, or a jumped timing chain, which will keep air from entering at all. As a note, air mixture issues will always have additional symptoms, such as loud noises in your engine bay or a loss of the power steering system. The Fix: Diagnosing the cause of an air mixture problem is a little more involved than most backyard mechanics can handle. See a professional mechanic to get an air mixture issue fixed.

Crankshaft sensor issues Your crankshaft sensors monitor your engine’s cylinders to ensure they’re rotating at the proper time. As a result, your vehicle probably won’t shake if the sensor is the only problem and the cylinders are actually firing at the right time. But if the sensor in your vehicle is somehow responsible for the cylinder firing, it could potentially cause your vehicle to shake. The Fix: The crankshaft sensor is usually kind of hard to access and calibrate, so see a mechanic to get this one repaired.

Problems with the emissions systems This is more likely to cause a solid check engine light than a flashing one, but it could flash depending on the underlying problem. Your emissions system is responsible for sending noxious fumes from the engine to the catalytic converter, but if there’s a leak or problem with the system, those emissions may flow back into the engine and trigger the occasional misfire. The Fix: You may need something as serious as a new catalytic converter, or something as minor as a patch on a pipe. Since this repair requires dealing with toxic emissions, have a mechanic do this one.

Can I drive with a flashing check engine light and shaking?

No, continuing to drive is dangerous for you and your vehicle. A misfiring engine is not a minor automotive issue. Your engine cannot safely continue to run if it’s misfiring, and continuing to drive may destroy your engine permanently. You could also lose power at any moment if you keep driving, which can lead to risky situations on the road. If this happens again when you’re driving, put your hazards on and gently coast to the curb. Once you can safely stop, turn the vehicle off and call a tow truck.

How to Diagnose the Exact Problem

Plug an OBD-2 scanner into your vehicle and run a scan. Turn the vehicle off and locate the OBD-2 port under your steering wheel (it’ll be behind a panel in most cases). Plug an OBD-2 scanner into the port and turn it on. Then, turn your vehicle to the “on” position. Press “run” or “scan” on the OBD-2 scanner and wait for it to return a diagnostic code. That code will refer to a specific error. Look up the error in your OBD-2 scanner’s manual to determine what the specific problem is.