A flashing check engine light is your car’s way of telling you there’s a serious problem and you must stop as soon as possible. It is often due to a misfire in the engine or a backfire from improperly timed ignition. However, there are several other potential problems, none of which are safe to ignore.
In some cars, the way the check engine light flashes can give you a clue as to where the problem lies. The wise move is to pull over and check the diagnostic codes with a quality scanner.
How Does the Check Engine Light Work
The check engine light is an integrated part of a car’s onboard diagnostics system. The ECU or PCM coordinates it and simultaneously collects performance data on the car’s engine, transmission, and various sensors.
1. Data from Sensors and Data
Most modern cars are engineered with various sensors that actively monitor the engine, transmission, emissions, braking, traction control, and exhaust system. Most of these sensors have safe operating parameters.
This includes major things like the fuel/air ratio, the composition of the exhaust gases, fuel vapors, engine temperature, and transmission performance. The wheel speed and other sensors in the braking system also send data that the traction control and anti-lock braking system use to improve the handling of abnormal road conditions.
2. The ECU or PCM
Some cars have a PCM (Power Train Control Module) that simultaneously monitors and adjusts the engine and transmission performance. Some cars have a separate ECU (Electronic Control Unit) and/or a TCM (Transmission Control Module) that monitors and adjusts the engine and transmission respectively.
The PCM/ECU actively processes data to adjust engine and transmission performance in real-time. It also helps improve handling and reduce emissions.
Anytime a sensor sends an intermittent signal, fails to send a signal, or detects performance outside of the established safe operating parameters, the PCM/ECU will trigger the check engine light.
Yet another set of parameters will determine whether it flashes the check engine light or turns it on and leaves it on solid.
Diagnostic Trouble Codes
Anytime the check engine light comes on, the PCM/ECU will “Throw” and store a specific diagnostic trouble code (DTC). Each code directly corresponds to a specific component malfunction or problem with the engine, transmission, EVAP, cooling, exhaust, braking, or traction control system.
The code will remain stored in the car’s computer whether the check engine light is blinking or stays on. You can check it with an OBD II scanner or similar diagnostic tool.
Things That Cause a Flashing Check Engine Light
The two most common causes of a flashing check engine light are an engine misfire or a backfire in the exhaust system. Depending on the model, the check engine light might only blink for a split second after the misfire occurs and then stop.
Other mechanical faults can trigger a flashing check engine light. In some cars, the frequency or pattern of the blinking check engine light can clue you in on what system is having a problem.
1. An Engine Misfire
Engine misfires are the most common trigger for a flashing check engine light. The untimed detonation of fuel occurs outside of ECU parameters, and the light flashes to warn you of a serious problem.
The check engine light might only blink for a split second as the misfire occurs. I had a truck where the check engine light would flash for a fraction of a second when a misfire occurred. I noticed it happened a handful of times. Then, I hooked it up to a scanner and found over 20 stored misfire codes that I must have missed.
Diagnostic Codes for a Misfire
A flashing check engine light due to an engine misfire can store a variety of codes that can be confusing depending on the root cause. However, there are a few to keep a keen eye out for.
2. An Exhaust Backfire
A flashing check engine light from a backfire usually happens when unburned fuel from a misfire seeps into the exhaust system. It then ignites with the heat of the exhaust manifold or the catalytic converter with a massive bang and vibration.
Sometimes, the misfire can be so severe that a short burst of flame comes out of the tailpipe. The check engine light might blink twice in succession, once for the engine misfire and again when the oxygen sensors respond to the backfire in the exhaust system.
Exhaust backfires like this can cause serious damage throughout the exhaust system, causing thousands of dollars of damage.
Diagnostic Codes for an Exhaust Backfire
A flashing check engine light from an exhaust backfire will store a lot of the same codes you’ll get from a misfire. This makes sense, considering the backfire is an after-effect of unburned fuel from a misfire migrating into the exhaust system.
Though you might get some other codes to help clue you in.
Code P0420 is a general catalytic converter code that can also indicate a bad upstream oxygen sensor. It will also come up if unburned fuel from a misfire causes a backfire that affects the catalytic converter.
Codes P0172 and P0175 are generic trouble codes that get stored when the engine’s fuel/air mixture is too rich. The excess unburned fuel passes into the exhaust system, where it ignites. You usually see this with a stuck open fuel injector or related fuel system issues.
3. An Engine Backfire
A flashing check engine from an engine backfire can happen when the internal combustion process is disrupted in one or more of the cylinders. Instead, the fuel/air mixture combusts in the intake manifold or carburetor.
Timing issues cause many engine backfires. These include a malfunctioning ignition coil, multiple fouled spark plugs,or a faulty timing belt.
Codes for an Engine Backfire
A blinking check engine light with an engine backfire is very similar to a misfire and will give you a lot of the same codes. You’ll almost always get a P0300 indicating multiple cylinders failed to fire. There are also some other codes to keep an eye out for.
Code P0016 or P0017 indicates the crankshaft and camshaft position sensors are out of sync.
Code P0340 indicates a problem with the camshaft position sensor. This can be due to a loose wiring issue, a bad connection, or a failure in the sensor itself.
Code P0344 indicates a problem with the crankshaft position sensor.
Pro Tip
If the engine backfires due to a problem with the timing chain or belt, you’ll often hear a clattering or rattling noise from the chain slopping around and making contact with the timing cover. It might be more pronounced when you accelerate hard, the engine will seem to bog down, then rattle, and suddenly backfire in sequence.
4. A Vacuum Leak in the Brake Booster
If the check engine light flashes and the engine stumbles for a moment when you step hard on the brake pedal, you might have a vacuum leak in the brake booster. This is one of the strange and rare reasons for a blinking check engine light.
Many cars have a vacuum line that runs from the engine to the brake booster. If the line or the diaphragm inside the booster leaks, unmetered air can enter the engine’s air intake.
The engine stumbles due to an imbalance in the fuel/air mixture. When you brake hard or for a long time, the imbalance can exceed the ECU’s acceptable threshold, causing the check engine light to turn on momentarily or flash.
Codes for a Brake Booster Vacuum Leak
A flashing check engine light from a bad brake booster typically throws codes indicating that the engine is running lean.
Codes P0171 and P0174 are general codes indicating that the engine is running in a lean condition.
Codes P0555 through P0559 indicate a problem with the brake booster.
A vacuum leak in the brake booster will also affect the braking performance. The pedal will feel spongy when you step on it and will usually rebound slowly. Since less mechanical force will be transmitted to the master cylinder, the car will suffer a noticeably longer braking distance.
5. An Overheating Engine
In some cars, the check engine light will flash and ding as the car overheats. This might also activate the engine temperature warning light.
The blinking check engine light is usually meant to alert you as the engine coolant temperature approaches the danger zone. If you ignore it or the light comes on and stays on, you’re headed toward a dangerous overheating event.
Codes for an Overheating Engine
Cars that flash the check engine light to warn of impending overheating problems might throw a variety of codes. Excess engine heat affects engine performance, so you’ll usually get the engine temperature warning light, and the ECU will store a general overheating code to keep an eye out for.
Code P0217 is a general code that stands for “Engine Coolant Over Temperature Condition.
Frequently Asked Questions
Is it safe to continue driving when the check engine light stops flashing?
If your flashing check engine light stops, the wise move is to pull over as soon as possible. It’s most likely a potentially dangerous misfire or backfire. The root cause will likely happen again, and there’s a high risk of serious engine damage each time.
How can I read check engine light codes if I don’t own a scanner?
These days, most auto parts stores will hook your car up to a scanner and tell you the diagnostic warning codes for free. They hope that if you can fix it on your own, you’ll buy the parts from them or have their in-house mechanics repair it. However, you’re not legally obligated to do so.
What’s the difference between a flashing check engine light and a solid light?
A flashing check engine light is usually a serious issue that can cause severe damage to the engine, cooling, or exhaust system. It’s usually related to a major malfunction, such as a severe misfire or a fault that can cause critical overheating damage. The wise move is to pull over and not drive the car any farther until you can find out why the check engine light blinks.
A solid check engine light usually indicates a less severe yet still serious problem. This can include minor problems like an EVAP system problem or a sensor that’s sending inconsistent data. You can usually drive with a solid check engine light until you can get somewhere safe.
Just keep an eye on the light. If it starts out solid but then starts blinking, the problem has worsened, and the car is no longer safe to drive.
Conclusion
A flashing check engine light is almost always the car’s way of warning you of a dangerous problem. You need to pull over as soon as possible or you risk major damage to the engine, exhaust system, or other critical components.
If you have a scanner tool or an OBD II reader, you can check the codes stored in the car’s computer when the check engine light is flashing. Yet the wise move is still to get the car towed.
Other more quirky mechanical failures can cause a blinking check engine light. If you’re lucky, it’s something as simple as a vacuum leak in the brake booster, which also causes the engine to stumble when you step on the brake pedal.
Written By
Jason Farrell
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.