The check engine light is one of the most misunderstood lights on the dashboard, and one detail matters more than most drivers realize: whether the check engine light is flashing or solid. A steady light is the engine computer saying it found a problem worth checking soon. A flashing light is the same computer telling you that damage may be happening right now. Here is how to tell the difference and what to do about each.
In both cases, the fix starts with finding the cause, which is what our check engine light diagnostics are for.
What the light is for
Your car’s engine computer constantly monitors sensors and runs self-tests on systems that affect emissions and engine operation. When a reading falls outside its expected range, it stores a diagnostic trouble code. If the fault meets certain criteria, it turns on the check engine light, sometimes labeled “service engine soon” or shown as an engine-shaped symbol.
It is normal for the light to come on briefly when you first turn the key, as part of a bulb check. What matters is whether it stays on once the engine is running, and how it behaves.
A solid check engine light
A steady light means a fault has been detected, but the computer does not consider it an immediate threat to the engine or catalytic converter. Common causes include:
- A loose, damaged or missing gas cap
- A small leak in the evaporative emissions system
- A failing oxygen sensor or mass airflow sensor
- A thermostat that opens too early
- Worn spark plugs or an intermittent ignition problem
- A catalytic converter that is losing efficiency
A solid light is not an emergency, but it is not something to ignore for months either. Problems that start small can affect fuel economy, cause other parts to wear, and while the light is on for one fault, a second problem would not trigger a new warning. Schedule a diagnosis soon, and pay attention to any change in how the car drives. If you are wondering whether to keep driving in the meantime, our article is it safe to drive with the check engine light on covers that question in more depth.
A flashing check engine light
A flashing light most often means the engine is misfiring badly enough to damage the catalytic converter. When a cylinder misfires, unburned fuel goes into the exhaust. Inside the hot converter, that fuel can burn and push temperatures far above normal, which can melt the converter’s internal honeycomb. Converter replacement is a significant repair, and on some vehicles there is also a fire risk under the car from extreme heat.
Common causes of a severe misfire include:
- A failed ignition coil or spark plug
- A stuck or leaking fuel injector
- A large vacuum leak
- Low compression in a cylinder from a mechanical problem
What to do when it flashes
- Reduce load right away. Ease off the gas, avoid hard acceleration and do not pull a trailer or climb steep grades if you can avoid it.
- Watch for other symptoms. Heavy shaking, loss of power, a fuel smell or a hot burning smell are reasons to pull over somewhere safe and shut the engine off.
- Keep the trip short. Head to a safe stopping point or a repair shop, not across the state.
- Get it diagnosed promptly. The longer a severe misfire continues, the greater the risk to the converter.
If the converter has already been damaged, our exhaust and catalytic converter service can inspect it and explain your options.
When the light changes behavior
Sometimes a flashing light turns solid, or a solid light goes out on its own. That usually means the condition that triggered it has eased, not that the problem is gone. The code is typically kept in memory, so a technician can still read it. Do not clear codes or disconnect the battery before your appointment; that wipes out useful information. Our guide on how mechanics diagnose car problems explains why that data matters.
Other lights that need more urgency
The check engine light is amber for a reason. Red warning lights for oil pressure, engine temperature or charging problems generally call for stopping as soon as it is safe. Check your owner’s manual for what each light means on your vehicle.
Have the light checked in Hermitage
Flashing or solid, Music City Auto Repair will read the codes, test to confirm the cause and explain your options with an estimate before any work begins. Request an appointment online or call 615-600-5994.
Frequently asked questions
Why does the check engine light come on briefly every time I start the car?
That is a normal bulb check. When you turn the key or press the start button, most dashboard warning lights illuminate for a moment to show they work, then go out once the engine starts. If the check engine light stays on after the engine is running, a fault has been stored.
Why did my check engine light turn off by itself?
Some faults are intermittent. If the engine computer no longer sees the problem over several drive cycles, it may turn the light off, but it usually keeps a record of the code in memory. A problem that comes and goes is still worth having diagnosed, especially if it returns.
What should I do first if my check engine light starts flashing on the interstate?
Ease off the accelerator, avoid hard acceleration and heavy loads, and move to the right lane. If the engine is shaking badly, losing power or you smell something hot, exit and park somewhere safe. Have the vehicle diagnosed as soon as possible rather than continuing a long trip.
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