Check Engine Light Comes On Then Goes Off

Quick Answer

When the check engine light comes on briefly and then turns off, the engine's computer detected a fault condition that later corrected itself. The light turns off because the computer ran its diagnostic test again and the problem was no longer present. However, the diagnostic trouble code (DTC) that triggered the light is still stored in the computer's memory as a "pending" or "history" code. Reading that code with an OBD-II scanner tells you exactly what was wrong, even if the light is off now.

What the Computer Is Doing

Your car's engine control module (ECM) runs hundreds of diagnostic tests continuously while you drive. It monitors sensor readings, compares them to expected values, and flags anything that falls outside the acceptable range. When a test fails, the ECM stores a diagnostic trouble code and turns on the check engine light (formally called the Malfunction Indicator Lamp, or MIL).

Here is the nuance: most faults must occur on multiple consecutive drive cycles before the light turns on. And the light turns off automatically if the same test passes for a certain number of drive cycles after the fault. This is why the light can appear for a day or two and then vanish.

The light going off does not mean the problem is fixed. It means the problem was not detected during the most recent set of tests. It could be:

A genuinely intermittent problem that only occurs under specific conditions (temperature, humidity, engine load, RPM)
A one-time event that is unlikely to recur (a bad tank of gas, a momentary sensor glitch)
A developing problem that has not yet become consistent enough to keep the light on

The stored code tells you which category you are dealing with.

How to Read the Stored Code

Even with the light off, the code is stored in the ECM's memory. You can read it with:

An OBD-II scanner. A basic OBD-II scanner costs $20 to $50 and plugs into the diagnostic port under the dashboard (usually to the left of the steering wheel, near the knee area). Turn the ignition to ON without starting the engine, and the scanner will display stored and pending codes.

A smartphone app and Bluetooth adapter. Apps like Torque (Android) or Car Scanner (iOS) work with a $10 to $20 Bluetooth OBD-II adapter. Same process -- plug in, connect via Bluetooth, and read codes.

An auto parts store. Most major auto parts stores (AutoZone, O'Reilly, Advance Auto Parts) will read your codes for free. Drive to the store, and an employee will plug in a scanner and give you the code numbers and basic descriptions.

The code itself is a letter followed by four digits (like P0171 or P0420). The letter indicates the system (P = powertrain, B = body, C = chassis, U = network). The numbers narrow down the specific fault. A quick web search for the code plus your car's make, model, and year will give you detailed information about what the code means and the common fixes.

Common Codes That Come and Go

Some trouble codes are known for being intermittent:

P0171 / P0174 -- System Too Lean. The engine is getting too much air or not enough fuel. Common causes: a small vacuum leak that only opens when the engine is hot, a MAF sensor that is slightly dirty, or a marginal fuel pump. If the car hesitates when accelerating, this code may be related.

P0420 / P0430 -- Catalyst System Efficiency Below Threshold. The catalytic converter is not performing as well as the computer expects. On many vehicles, this code appears intermittently for months or years before becoming permanent. A failing catalytic converter, aging oxygen sensors, or a small exhaust leak can trigger it.

P0300 through P0308 -- Random / Cylinder-Specific Misfire. The engine misfired. P0300 means random misfires across multiple cylinders (often a fuel quality issue). P0301 through P0308 point to a specific cylinder. Intermittent misfires can be caused by a spark plug that is starting to fail, a marginal ignition coil, or a fuel injector that sticks occasionally.

P0440 / P0442 / P0455 -- Evaporative Emission System Leak. The system that captures fuel vapor detected a leak. This is frequently triggered by a loose or worn gas cap. Tightening or replacing the gas cap ($5 to $15) often resolves it. If it returns, there may be a crack in a vapor hose or the purge valve.

P0128 -- Coolant Thermostat Temperature Below Regulating Temperature. The engine is not reaching operating temperature fast enough. Common in winter, especially with a stuck-open thermostat. The light may appear on cold mornings and turn off as the season warms up.

Should You Worry?

The answer depends on the code and the pattern.

One-time event, no driveability symptoms: A code triggered once by a bad tank of gas, a loose gas cap, or an extreme temperature is generally nothing to worry about. Read the code, note it, and see if it comes back.

Recurring pattern: If the light comes on every few weeks, stays for a day, and turns off, the problem is intermittent but persistent. It will likely become permanent eventually. Getting it diagnosed while it is still intermittent is often cheaper than waiting for the full failure.

Driveability symptoms present: If the light coincides with rough running, hesitation, reduced fuel economy, or a noticeable smell, the fault is affecting how the car runs. Get it diagnosed even if the light turns off, because the underlying issue is present and affecting performance.

Flashing check engine light: A flashing light (as opposed to a steady light) means an active, severe misfire that is damaging the catalytic converter right now. If the light was flashing and then went off, the misfire was severe but temporary. Read the code immediately and get it checked. Catalytic converter damage is expensive ($1,000 to $2,500).

Clearing Codes vs. Letting Them Clear Naturally

You can clear stored codes with an OBD-II scanner by selecting "Clear Codes" or "Erase DTCs." This turns off the light immediately and erases the stored codes. Some people do this to see if the light comes back.

There is a downside: clearing codes also resets the readiness monitors -- the computer's record of which diagnostic tests have completed. If your state requires emissions testing, the car will fail until all monitors have run and completed, which can take several drive cycles (50 to 100 miles of varied driving). If you have an upcoming emissions test, do not clear the codes unless you have time to drive enough cycles to reset the monitors.

If you leave the codes alone, the computer will turn off the light on its own after a set number of passing drive cycles (usually 3 to 5), and the readiness monitors remain intact.

If your car also has issues like rough idling alongside an intermittent check engine light, the stored code will often connect the two symptoms to a single root cause.