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Check Engine Light Flashing: Reasons Explained

Car problems always seem to happen at the most inopportune moments, not that there is ever a good time.

Fortunately, cars have an inbuilt early warning system that can alert the driver to imminent trouble enabling them to arrange an inspection or repair before the cost escalates.

The check engine light flashing might fill you will dread, but is often an easy fix, providing the work is carried out immediately.

What is a Check Engine Light (CEL)?

check engine light

Not everything that goes wrong with a car can be seen or heard; some components that malfunction are more difficult to detect. Certain problems will only become obvious via the check light. A faulty oxygen sensor is a prime example; the only way of detecting the issue is when the light switches on.

The check engine light is part of a vehicle's onboard diagnostic system. When the ECU or Engine Control Unit detects a problem that it is unable to regulate, it powers the light to alert the driver.

Where is the Check Engine Light Located?

It is located on the dash and often has an engine graphic. On some makes of vehicle, the graphic is replaced with either 'check engine' or 'service engine.'

Not all warning lights flash, some check engine lights illuminate red when there is imminent danger that requires immediate attention. Yellow lights warn of less sinister issues that should be resolved as soon as it is possible.

Solid Check Engine Light

If the light remains on and doesn't blink, this is usually the sign of a less urgent problem. If you're really lucky, it might just be that the fuel cap hasn't been properly tightened. It might also be an alert of a timing, fuel, or transmission issue, whilst they require attention, the problem isn't as severe as when the check engine light is flashing.

Flashing Check Engine Light

A blinking light is an emergency. The driver should head for the closest auto shop to get the problem diagnosed and repaired with some haste. If possible, consider having the car towed.

How to Diagnose a Flashing Check Engine Light

man using obd2 scanner

The easiest way to diagnose the problem is to have a professional mechanic use a diagnostic tool to scan for fault codes.

However, OBDII scanners are relatively inexpensive and make it possible for you to diagnosis the error at home.

Most OBD scanners connect to the car via a port usually found beneath the steering column. With the ignition switched on, the scanner reads all of the engine's systems, and any faults are shown on the screen as a trouble code.

Cross-reference these codes with those found in the User Manual to find the issue.

Common Causes of a Flashing CEL

Loose or Faulty Gas Cap

As crazy as it may seem, millions of dollars worth of fuel is lost in the USA each year due to evaporation. Take heed if you check engine light switches on, your fuel cap might need to be tightened. If it has a crack, replace it.

Average repair cost: $0 - $5

Failed O2 Sensor

emissions test

A critical part of the exhaust system is the O2 Sensor. It's located inside your exhaust pipe and monitors the amount of oxygen in the exhaust.

The fuel mixture can either be too lean (too much oxygen) or too rich (too little oxygen), either will cause the O2 Sensor to send a signal to the ECU and populate the CEL.

Driving with a faulty sensor will reduce fuel economy and damage the catalytic converter.

Average repair cost: $150

Damaged Catalytic Converter

Often an avoidable and very expensive repair caused by ignoring smaller issues. It is an integral part of the exhaust system made from expensive metals.

Average repair cost: $900 - $1200

Spark Plugs/Wires

Replacing spark plugs might not seem an emergency job, and rightly so. However, damaged spark plugs and wires will lead to misfires and can escalate to much bigger and more expensive to repair problems.

Should issues spread to the ignition coils or catalytic converter, then repairs run into several hundreds of dollars. Repairing plugs and wires depends on the vehicle and the accessibility of the work.

Average repair cost: upwards of $100

Faulty Air Flow Sensor

The airflow sensor calculates the exact amount of fuel to mix with the oxygen within the engine, preventing it from running too rich. When the sensor fails, the fuel economy is reduced by up to 25%

Average repair cost: $250 - $450

Evaporative Emissions Control

The fuel tank and fuel system within a vehicle that prevents vapors from escaping into the atmosphere. It is made up of many components, each of which triggers the check engine light should they malfunction.

  • Vent control valve: $249
  • Canister vent valve: $208
  • Solenoid: $221
  • Purge solenoid: $196
  • Purge control valve: $176

Many other problems activate the check engine light causing it to flash. Most of them are covered here, including their average repair costs:

  • Replace crankshaft position sensor: $220
  • Replace intake manifold gasket: $420
  • Engine coolant temperature sensor: $175
  • Throttle body assembly: $590
  • FTP, fuel tank pressure sensor: $350
  • Camshaft position sensor: $220
  • Fuel injectors: $480
  • Thermostat: $240
  • Ignition coils: $265
  • Replace and clean EGR valve and ports :$360

How to Reset a Flashing CEL

If the repair has been carried out in an auto shop, the light will have already been reset before you receive the vehicle.

If you have completed the work yourself there are four ways to reset the light.:

  1. Reconnect the OBD2 scanner and press the Erase/Clear button. This should work immediately, if it doesn't, switch the engine off and on before trying again. If it still doesn't go off run a diagnostic cycle to check that there are no new issues.
  2. If you don't have a scanner, disconnect the battery. Remove both the positive and negative terminals and leave them off for 15-minutes. Re-attach them and start the engine to check that the light has gone off.
  3. Repetitive on/off of the ignition sometimes reset the light. Turn the key at 1-second intervals.
  4. If you are sure the repair has been completed, you can leave the light on. After 3-days the engine cycle will automatically reset the light. If the light remains on after 3-days you will need a diagnostic test on the ECU to detect any further issues.

Is it Safe to Drive with the CEL Flashing?

It is thought that at any one time there could be 10% of motorists in the USA driving with their check engine light on.

Even in situations where the repair isn't an emergency, the quicker the issue is resolved the greater the possibility is of avoiding an expensive repair.

Your vehicle might overheat, in which case, pull over to the side of the road immediately, turn the engine off and stand outside until help arrives.

Misfires might not seem too worrying but they may impact your ability to accelerate, resulting in you becoming a hazard to other road users.

If the CEL switches on and coincides with the drive becoming jerky, vibrating, or any unusual noises, pull over and get help.

What Happens When You Drive With the CEL Flashing?

If it is due to the failure of the oxygen sensor or the mass flow sensor, you will notice a significant reduction in mpg. Emissions will increase, possibly outside of your state's legal requirements. They might also emit levels severe enough to pollute the air and harm the environment.

Ignoring spark plug issues can become very costly. Replace them as soon as possible to prevent catalytic converter damage.

Ignition fuel in the cylinder and leaking fuel injectors enable excessive or unburned fuel to enter the catalytic converter. This raises its temperature above the tolerance level resulting in the melting on the inside. This excessive heat can also harm other parts of the engine.

Early intervention is imperative to prevent a few hundred dollars from turning into a few thousand dollars.

If you have no choice but to drive to the dealership or repair shop then do so tentatively. Use the throttle lightly so as not to put the catalytic converter under additional pressure.

Final Thoughts

Once you notice the check engine light flashing, use an OBD2 scanner to self-diagnose the problem, or get it to a professional who can do it for you.

Any issue with the powertrain of your vehicle will be highlighted at the earliest opportunity and should be acted upon.

Ignoring a quick and cheap spark plug repair can result in a much more expensive catalytic converter replacement down the line.

Richard Reed
I've been a General Manager of a moving company and I've also been a Professional Mover for over 30 years. I've driven flat beds, reefers, dropdecks, moving vans, heavy machinery, etc. In my time as a Mover I've driven over 1,000,000 safe miles. My days of moving and driving truck are past me but The Vehicle Lab allows me to share the knowledge I've gained over the past 40 years.
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