Check Engine Light Flashing and Car Shaking When Accelerating Or Stopped [Fixed!]

Check engine light flashing and car shaking problem whenever you try to accelerate your car or you stop a car at a traffic signal is a very stressful situation. When you see a check engine light flashing/blinking, it indicates that there is a problem with your car’s engine and you have to immediately stop your car to find the root cause. In this post, I’ll also discuss some common causes of a check engine light flashing and car shaking.

If the check engine light is flashing and the car is shaking, it usually indicates the engine misfires. This could be caused by an issue with the ignition system, fuel injection, spark plug, and vacuum leaks.

I’ve another helpful guide on check engine light on but car runs fine. You should also check that out.

A Check Engine light on a vehicle may indicate there’s an issue with one or more of the engine’s components. The light could be caused by a faulty ignition coil, a problem with a sensor, an issue with an engine computer, or any of several other possible causes.

If your vehicle model is after 1996, you need to have an OBD2 scan tool that can read all trouble codes stored in the engine’s memory. This will help you understand which area of your engine you should inspect first that is causing engine misfire.

Generally, users observe the P0300 code on the scan tool which indicates that misfiring is occurring in one of the cylinders. OBD2 tool can also code specific code like P0303 which means that a misfire has occurred in cylinder#3. In that case, swap the ignition coil and spark plug between cylinder three and another cylinder to see if the miss moves or stays where with cylinder#3.

What Causes the Check Engine Light and Car Shaking?

An engine misfire causes the check engine light flashing and car shaking. When there is an engine misfiring, the engine may stall, your vehicle may shake and it will experience a reduction in the power produced. Engine misfires are caused by one or more cylinders not firing properly, usually due to problems with the ignition system.

An engine misfire occurs when there is an incorrect combustion process in one of the engine cylinders.

For a smooth combustion process, there are the following conditions:

  • A perfect ignition timing
  • Suitable Air/Fuel Ratio

An engine goes through several phases of combustion to ensure that the engine is running smoothly and delivers optimized performance. The engine is programmed to inject fuel and ignite the spark plug at a certain position of the engine’s crankshaft. When one or more of the engine cylinders inside the engine doesn’t fire correctly because of an incorrect air/fuel ratio or delay in spark plug ignition, engine misfiring will take place.

Misfiring happens when the spark plugs fail to fire at the calculated time. The main cause of this problem is a faulty ignition coil. The ignition coil is used to provide a spark to the spark plugs.

It generates an electrical current and is connected to the spark plugs.  If the coil is damaged, it will not work correctly and will not produce a spark. This will cause the engine to misfire.

Engine misfire also occurs when the air/fuel mixture gets too rich or lean. If there is an incorrect air/fuel ratio, the combustion mixture does not burn fast enough. It burns in uneven pockets rather than uniform bursts. The uneven combustion produces shock waves that cause the car to shake.

So, the reason for the car shaking during engine misfiring and flashing check engine light is due to the engine knocking. When a combustion event does not occur at a certain moment, knocking occurs and causes vibrations in the engine.

Can Engine Misfire Damage Catalytic Converter?

Yes, the engine misfire also damages the catalytic converter. If your car is shaking and the check engine light is flashing, the OBD2 scan tool can show a trouble code related to the ‘Low catalytic system efficiency.

The purpose of a catalytic converter is to convert unburnt hydrocarbons, carbon monoxide, and NOx to less harmful gases. When there is a misfire in the engine, excessive unburnt fuel will pass through a catalytic converter where the fuel will undergo combustion. This will considerably increase the design temperature of the catalyst and damage it.

Due to excessive unburnt fuel, the catalytic converter will be clogged. When a catalytic converter becomes clogged, it restricts the exhaust flow. As a result, the exhaust gases will not be able to escape the cylinder before the new air/fuel charge is sucked into the cylinder. That’s why it is really important not to drive when the check engine light starts flashing and your car starts shaking.

Common Causes Of Check Engine Light Flashing And Car Shaking

1. Bad Fuel Injector

The fuel injectors in vehicles are designed to spray the fuel into the combustion chamber of the engine at a specific time. They are located under the vehicle hood and are controlled by the electronic module of the vehicle. 

Now, modern gasoline engines have adopted direct injection technologies that directly inject highly-pressurized fuel into the combustion chamber so that it readily atomizes and mixes with the air for even combustion. With time, fuel injectors become clogged with carbon deposits or also become stuck open, due to which they fail to inject atomized fuel at the right time.

When the fuel injector is bad, it can cause the mixture of air and fuel to be uneven, leading to problems with combustion. When the fuel injector malfunctions, it can cause a misfire. This is because the spark plug only ignites the air/fuel mixture when there is an even spread of fuel and air.

The problem of a faulty injector could also be caused by a weak electrical connection between the injector and the ECU.

There are different tests to check the fuel injectors. First, we have to check the O-rings on the top and bottom of the fuel injector as a leaky fuel injector will fail to deliver sufficient fuel to the engine cylinder and could cause a misfire. Next, we have to perform an electrical test on each fuel injector to see if it’s clicking or not. After that, a mechanical test is done on the fuel injectors to observe whether they are injecting fuel at a right angle.

I have written another detailed guide on car won’t start after replacing fuel injector. You should read that guide to learn about different tests done on a fuel injector. Moreover, in that guide, I have also discussed a procedure to check the electrical connection of the fuel injector.

2. Bad Ignition Coils

A bad ignition coil is one of the most common causes of check engine light flashing and car sputtering or shaking. Ignition coils supply high voltage to the spark plugs to initiate combustion in the engine. Ignition coils are a transformer that increases voltage by many times. An ignition coil consists of two types of winnings:

  • Primary winding
  • Secondary winding

The primary winding is connected to the 12V of the battery via the engine control module. The secondary winding is connected to the spark plug to generate a spark.

Now, the configuration of an ignition coil depends on the vehicle model. There are three configurations of an ignition coil:

  • Distributor system
  • Coil Pack
  • Coil On Plug (COP)

The distributor system has only one ignition coil controlling all ignition in all cylinders. Such distributor systems can be found in 4.3L Vortec engines. In a distributor ignition system, the primary winding is wrapped around the core, while the secondary winding is attached to the distributor cap.

This type of ignition system with a distributor is complex and difficult to maintain as we have contact breaker points in the distributor system that erode over time. Moreover, inside the distributor cap, carbon builds up over time, which also affects the resistance of the electrical path from the secondary coil to the spark plug. I found this document (PDF) really helpful to get an idea about all possible issues in the distributor ignition system.

In some engines, there is a coil pack instead of a distributor. It is also called the ‘Wasted Spark Ignition System’. Each coil pack consists of two or more coils. Each coil pack controls spark plugs ignition in two or more cylinders. So, if your engine has 6 cylinders, there will be three ignition coil packs. In the picture below, you can see a coil pack to control the sparks in the four cylinders. It has four terminals (two on each side) controlling the four cylinders.

coil pack in engine

The last type is Coil On Plug, which is present in modern engines. In this configuration, each cylinder has its ignition coil on which the spark plug is tightened. Such ignition coils have two, three, or four-wired configurations. You can check that by counting the pins in the port where the electrical connector is plugged. If you look at the ignition coil in the picture below, it is three-wired.

two-wired ignition coil

If the ignition coil is short at some point, it will cause intermittent car starting failures. To diagnose an ignition coil, you have to measure the resistance across positive and negative terminals of both primary and secondary coils of ignition coils in each cylinder.

You can find the optimal resistance values of an ignition coil in its specs or service manual. The resistance across terminals of the secondary coil is in thousands of ohms and resistance across terminals of the primary coil should be less than 1ohm. 

3. Bad Spark Plugs

Spark plugs, which create a spark in the engine to initiate combustion, can be a critical component of your engine. If they are not functioning correctly or are damaged, your engine may experience problems. These problems can lead to checking engine light flashing, engine shaking, or rough idle.

Spark plug becomes bad due to the following things:

  • The gap between the spark plug center (tip) and the side electrode is not properly sized. Maybe the electrode is pushed down.
  • Porcelain (white-coating of insulation) is not sealed properly. Due to poor fitting or worn insulation of spark plug, you will see black burn blacks that occur due to the current discharge from the top terminal of the spark plug, down the outside of the insulator.
  • Deposits of burning oil, ash, or fuel on the tip of the spark plug. Such deposits on spark plugs are built up due to imbalances in air/fuel ratio or bad fuel injectors.

I have found this amazing guide (bad spark plug signs PDF) that includes pictures of all possible damages with the spark plug. 

4. Bad Variable Valve Timing System (VVT)

In modern engines, there is a VVT system to control the opening and closing of engine valves at higher speeds. The timing of valves is controlled by a cam phaser that transfers motion from the crankshaft to the camshaft via a timing chain. There is a solenoid valve controlled by an engine control unit (ECU). When ECU sends signals to the solenoid valve, oil passes through it under high pressure to operate the cam phaser and adjust the opening or closing of engine valves.

You can follow the following YouTube video for the test of the VVT solenoid valve:

In addition to the VVT solenoid, a bad cam phaser also causes the engine to misfire and flashes the check engine light. You may also need to replace it.

5. Poor Oil Quality

Oil is a vital part of a vehicle’s engine. It lubricates and cools moving parts and helps protect the engine from wear and tear. A faulty or low-quality oil can cause engine problems. The oil your car uses must be the right type for your vehicle.

Oil operates control valves and helps keep the engine clean by preventing sludge. If you have not changed your engine oil for more than 5000 miles, you should consider changing the oil at first. You can use dipstick to see the color of the oil. A bad oil is dark or black in color and is thicker due to engine deposits. Also, make sure that there is optimum oil pressure in the engine. Check my guide on low oil pressure at idle for more details.

6. Bad Engine Sensors

Bad engine sensors (MAF, O2, CKP, Camshaft) can also cause check engine light flashing and car shaking due to engine misfire.

A car can have either MAF or MAP sensor. It depends on your engine. PCM uses readings for MAP and Throttle Position Sensor to measure the fuel amount to be injected. MAF sensor calculates the volume of airflow and sends this information to the ECU to measure the fuel amount. Air passes through the MAF sensor before entering the throttle body so that ECU can adjust the air/fuel ratio and opening of the throttle body.

The CKP sensor measures the position and speed of the crankshaft. ECU needs this information to control valve timing, ignition timing, and fuel injection. You can read my guide on car won’t start after replacing crankshaft sensor. O2 sensor calculates unburnt oxygen in emissions and sends this information to PCM to determine desired air/fuel ratio. You can read this guide to test the O2 sensor

7. Vacuum Leaks After MAF Sensor

Vacuum leaks in the engine can also cause the engine to misfire and shaking of your car. During the intake stroke, a vacuum is created in the engine that sucks the air into the cylinder. If there is any leakage in the intake manifold after the MAF sensor, outside air will enter the engine, which will not be detected by the MAF sensor. As a result, ECU will not be able to recognize that air. The mixture will run lean and misfire will occur. You can use carb cleaner to detect vacuum leaks in the engine.

8. Bad EGR Valve

A bad EGR valve can also cause car shaking and blinking of the check engine light. EGR stands for Exhaust Gas Recirculation. The EGR valve is used to reduce emissions by recirculating exhaust gas back into the combustion chamber. It helps reduce the combustion temperatures and the number of NOx gases in the engine exhaust. The temperature of combustion is reduced because less oxygen is needed from the air for combustion. 

A frequent problem with the EGR valve is that it is often stuck due to a build-up of carbon deposits. The EGR valve and the exhaust gas recirculation passages are blocked by the carbon deposits. Blocking the EGR valve can increase fuel consumption and reduce engine performance, which can turn on the check engine light.

9. Bad EVAP Valve

EVAP solenoid valve is used in the fuel emission control system. EVAP valve is a one-way electronically controlled solenoid valve that can cause engine misfire and car shaking if it is stuck open. When the EVAP valve is stuck open, excessive fuel vapors are sucked into the combustion chamber. The mixture becomes rich and causes incomplete combustion. If you have a Ford vehicle, you can see the P1450 code on the OBD2 scan tool if the EVAP valve is bad.

You can watch the below video to test the EVAP valve:

Is It Safe To Drive When Your Car Is Shaking?

It is not safe to drive a car when it is shaking and the check engine light is blinking. You can worsen the condition of your engine if you continue to drive your car in that condition. If you continue to drive if the check engine light is flashing, it will damage the piston, crankshaft, gaskets, seals, and catalytic converter of an engine.