Check Engine Light Flashing But No Codes: Fixed

If you’ve ever been in a situation where your Check Engine Light starts flashing at you, you know it’s not something to ignore.

The CEL is designed to be your car’s early warning system, a yellow flag that something is amiss in your vehicle’s complex machinery.

But what happens when that crucial warning system gives you a sign by flashing, no less, and then leaves you in the dark by not providing any diagnostic codes?

To say it’s perplexing would be an understatement. You’re not alone, though, and that’s why you’re reading this blog post today.

Just to give you an idea of how baffling this can be, let me share a recent experience with a buddy of mine. He owns a 2015 Mazda CX-5 2.5L AWD.

He called me up one day, clearly frustrated: “The check engine light came on yesterday, and I’m feeling a loss of power on hills and experiencing a rough start.

I tried scanning the vehicle, first with an inexpensive scanner from AutoZone. It cleared the check engine light but showed no code. Then, I took it to a friend with a high-end scanner, which even displayed no code. What’s going on?”

If this story sounds eerily similar to something you’re going through, then you’re in the right place.

Today, I will be explaining the perplexing issue of a flashing Check Engine Light with no accompanying codes.

I’ll guide you through why this happens, how to diagnose the issue effectively, and what steps you should consider to resolve it.

Diagnostic tool lying on a car seat

Reasons Why Your Check Engine Light Might Come On With No Codes

Let’s go over some of the reasons why this may be happening. You might find it’s a simpler explanation than you thought.

1. Scan Tool Compatibility

First things first, not all scan tools are created equal. You might think that any OBD-II scanner out there should be able to read your car’s diagnostic codes, but that’s not necessarily the case.

Many vehicle makes and models have specific requirements when it comes to scan tools.

Always ensure that the scanner you’re using is compatible with your car. In the case of my buddy with the Mazda CX-5, he first used an off-the-shelf scanner, which was not suitable for his car’s specifics.

Trust me, spending a bit more on a compatible device is worth avoiding the headaches later on.

2. Timing

Sometimes, there’s a delay between when the CEL starts flashing and when the code actually gets stored. This delay can make it seem like there’s no issue, but that’s misleading.

If you’re in this boat, keep the scanner connected for a while longer or cycle the ignition a couple of times. Wait a few moments, then check the scanner again to see if any codes appear. Patience can sometimes solve the mystery.

3. Pending Codes

These codes indicate a problem the car’s computer is still evaluating. Until the computer confirms the issue, the codes remain “pending,” and some scanners won’t display them.

If your scan tool has the option to show pending codes, make sure it’s enabled. If not, you might want to invest in a more advanced tool or, better yet, consult a professional who has one.

4. Intermittent Problems

One of the trickiest scenarios is dealing with intermittent problems. Let’s say your car behaves badly only occasionally. Because these issues are not constant, they can cause the CEL to flash but might not store a code. It’s like having a ghost in the machine.

My advice here is to monitor your car’s behavior closely. Note down when the light flashes, under what conditions, and any other symptoms you observe. This information can be very crucial when diagnosing the issue, either for yourself or when you take it to a pro.

5. Advanced Codes

These are manufacturer-specific codes that generic scan tools might not pick up. In some cases, you might need a specialized tool or software that’s designed for your vehicle’s brand to diagnose the issue.

Sometimes, these manufacturer-specific codes can only be read at a dealership or a specialized mechanic shop. If you’ve tried all else and are still stumped, it may be time to consult the experts who have access to specialized diagnostic tools like OEM-enhanced scan tools.

Potential Issues That Can Trigger Your Check Engline Light With No Codes and How To Fix Them

1. Faulty Sensors or Wiring

Begin with a visual inspection of the wires leading to and from sensors related to the engine, such as the oxygen sensors, coolant temperature sensor, and the throttle position sensor.

Look for signs of frayed wiring, corroded connectors, or dislodged pins. If everything appears fine visually, use a multimeter to check the integrity of the sensor circuits.

If you find a damaged wire, use wire strippers to remove the damaged section, then splice in a new section of wire using a butt connector.

If a sensor seems to be at fault, most sensors are plug-and-play and can be easily screwed in and connected.

Ensure that recalibrate them according to the manufacturer’s instructions. This might require a specialized diagnostic tool.

But then, I highly suggest you leave this to a professional to handle if you’re not familiar with it.

2. ECU Problems: ECU/ECM/PCM

Errors in the Engine Control Unit (ECU), Engine Control Module (ECM), or Powertrain Control Module (PCM) can be elusive.

Look for symptoms like poor engine performance or inefficient fuel usage. To confirm, you’ll need a diagnostic tool or an ECU Stimulator capable of interfacing with your car’s computer systems.

If it’s a software issue, an update using specialized software from your vehicle’s manufacturer might fix the problem.

Hardware replacement can be complex and may involve reprogramming. This job is generally left to professionals unless you’re skilled in automotive electronics.

3. Bad Ignition Coils, Distributor Caps, or Coil Packs

Signs of bad ignition coils or related components include engine misfires, decreased fuel efficiency, and sluggish performance.

You can use an ohmmeter to measure the resistance across the coil and compare it to factory specifications found in your service manual.

To replace these, first, disconnect the car battery. Remove the defective coil pack or distributor cap, often held in place by a series of bolts or clips.

Install the new part and reconnect it precisely as the old part was connected. Reconnect your battery and start the car to ensure the issue is resolved.

4. Spark Plug is Not Properly Seated or Damaged

Symptoms include poor acceleration, engine misfires, or an irregular idle. Use a spark plug wrench to remove the spark plug and examine it for wear, carbon deposits, or damage.

Replacing a spark plug involves removing the old one with a spark plug socket and installing a new one. Be cautious not to overtighten, as that could damage the cylinder head. If the existing spark plug was unseated, you could re-tighten it.

5. Damaged Fuel Injectors

Noticeable symptoms are a rough idle, stalling, or decreased fuel efficiency. Perform a fuel pressure test for a more definitive diagnosis.

To replace a fuel injector, relieve the fuel system pressure, then disconnect the electrical connectors. Remove the injector rail and replace the damaged injector(s). Reassemble everything and perform a pressure test to ensure it’s correctly installed.

6. Faulty Fuel Pressure

Black exhaust smoke or fouled spark plugs are indicators. A fuel pressure gauge can give you a definitive answer.

You’ll need to relieve fuel system pressure first. The fuel pressure regulator is often located on the fuel rail and is generally held in place by fasteners or clips. Replace it and reassemble the fuel lines and connectors.

7. Faulty Electrical Connections

Look for symptoms like intermittent power losses or random warning lights. A visual inspection can reveal loose connectors or frayed wires.

Depending on the issue, this could be as simple as tightening a loose connector or as involved as replacing part of the car’s wiring harness.

Make sure the power is disconnected before you begin any electrical work.

8. Blown Fuse Affecting ECU/ECM/PCM

If a fuse related to the ECU/ECM/PCM blows, it could disable those systems without necessarily providing a diagnostic code.

Consult your vehicle’s manual to locate the relevant fuse box and identify the specific fuse.

Once you’ve located the blown fuse, replace it with a new fuse of the same rating. This usually requires nothing more than a pair of needle-nose pliers.

How to Diagnose the Issue

Essential Tools for Diagnosis

  1. OBD-II Scanner: This is the first tool you should have on hand. It’s used to pull codes from your car’s computer.
  2. Multimeter: Essential for checking the electrical values of various components. It can tell you whether electrical parts like sensors and wires are functioning correctly.
  3. Specialized Software: Some car models have proprietary systems requiring specialized diagnosis software. These are usually available from the vehicle manufacturer or third-party suppliers.
  4. Service Manual: Your car’s service manual contains vital information on troubleshooting various issues, so it’s good to have it accessible.

Initial Code Scan

  1. Using the OBD-II Scanner: Connect the OBD-II scanner to the port usually located under the dashboard. Turn your ignition to the “On” position to power up the scanner. Follow the scanner’s instructions to read any diagnostic trouble codes (DTCs).
  2. If No Codes Are Showing: This could be due to a number of reasons we’ve discussed before, such as compatibility issues, intermittent problems, or pending codes. If no codes are showing, don’t clear the Check Engine Light yet, as it might erase pending codes.

Physical Inspection

  1. Ignition System: Visually inspect your spark plugs, distributor cap, ignition coils, and wires. Look for obvious signs of wear, corrosion, or damage.
  2. Fuel System: Check the fuel injectors, fuel pump, and fuel lines for any visible damage or leaks. Sometimes, a physical inspection can reveal issues that electronic diagnostics miss.

Sensor and Wiring Check

To check if a sensor is working, set your multimeter to the appropriate setting (usually DC voltage for most automotive sensors) and measure its output according to the specifications in the service manual.

You can also check the integrity of the wiring by setting the multimeter to measure resistance (ohms) and then checking the continuity of the wires.

A sensor that is outside of the manufacturer’s specified range is likely faulty. For wiring, look for a reading that indicates an open circuit, which means the wire is broken somewhere.

Advanced Diagnostics

  1. Compression Tests: These are used to diagnose the internal health of your engine. If simpler tests and inspections haven’t yielded results, a compression test can reveal issues like worn piston rings or burnt valves that won’t necessarily trigger a code.
  2. Manufacturer-Specific Scanners: If you’ve come up empty with a generic OBD-II scanner, it may be time to move on to a manufacturer-specific tool. These scanners can read advanced codes and offer more detailed diagnostics.


Navigating a flashing Check Engine Light without any diagnostic codes can be perplexing, but it’s not a dead-end.

The first step is to initiate a scan with an OBD-II scanner that is compatible with your vehicle. If no codes are displayed, resist the urge to clear the CEL, as this may erase any pending codes that haven’t yet registered.

A physical inspection of key systems like ignition and fuel is the next logical step. This often reveals issues that electronic scans miss.

Utilizing a multimeter can further assist in diagnosing faulty sensors or wiring problems. Advanced diagnostic tools and procedures may be required in more complex situations, including manufacturer-specific scanners and compression tests.

I can’t stress enough how crucial it is to address a flashing CEL promptly. Ignoring it can lead to serious engine damage that could require costly repairs down the line.

Equipping yourself with the right tools and knowledge can often help you nip these issues in the bud, saving you both time and money.