Why is the check engine light on? Steady yellow vs. flashing

When does the check engine light come on?

On most cars, the check engine light (engine warning light) is a yellow or orange engine symbol. The engine computer monitors the ignition, the fuel mixture, and exhaust and emissions-related systems throughout the drive. When a value goes outside its expected range, or one of its own tests fails, it writes a fault code to its memory and turns the light on when needed.

Not every fault turns the light on right away. For some faults, the engine computer waits first; if the problem shows up again on the next drive, it turns the light on. So there can be a code stored in memory even while the light is off.

When you switch the ignition on, it's normal for the lights on the instrument panel to come on briefly; this is a bulb check. On most cars, the check engine light goes out once the engine is running. If it stays on after the engine has started, the engine computer may have recorded a fault.

The difference between a steady yellow light and a flashing one

A steady light shows that the engine computer has recorded a fault. The light alone doesn't tell you how serious the problem is: the same light can come on for a sensor fault or for a problem that strains the engine. The way to find out is to read the stored code and have the car checked at a workshop. Driving the car for a long time with the light on can let a small problem grow.

A flashing light is usually a sign of something more urgent. On most cars, it means the engine computer is seeing a serious misfire (a cylinder not firing properly). Fuel that doesn't burn in the cylinder can pass into the exhaust and burn in the catalytic converter, which can make the converter overheat and get damaged.

If the light is flashing, don't treat it as a warning that can wait: don't strain the engine, follow the instructions in your car's owner's manual and have the car checked at a workshop without delay. Even if the light is steady, do the same if there is misfiring, loss of power, shaking, an unusual smell or smoke while driving.

Red warning lights (for example oil pressure or engine temperature) are a separate matter; your car's owner's manual explains what each light means and what to do.

Examples of causes that can turn the light on

Many different fault codes can turn on the check engine light. A code shows which system the problem is in; it doesn't always tell you which part needs replacing. For example, a P0420 code may not mean that the catalytic converter needs replacing; a mechanic finds the cause.

A few examples of causes that can turn the light on:

• Misfire (a cylinder not firing properly): may be related to the spark plugs, an ignition coil or an injector (P0300 series).
• Catalytic converter efficiency below threshold: may be related to the converter, the oxygen (lambda) sensors or an exhaust leak (P0420).
• Mixture too lean: may be related to an air leak on the intake side, the air flow sensor or the fuel supply (P0171).
• Insufficient EGR flow: may be a blockage or a valve fault in the exhaust gas recirculation system (P0401).
• Soot build-up in the particulate filter (DPF) of a diesel car (P2463).
• Turbo boost pressure lower than expected (P0299).

The light went out: is the fault gone?

Not always. With some faults, if the problem doesn't recur over several drives, the engine computer can switch the light off on its own; the code, however, stays in the fault history for a while. An intermittent fault can turn the light on again when the same conditions come up.

Clearing the fault code doesn't solve the problem either. Clearing doesn't fix the fault: if the problem persists, the code and the warning light come back. The right order is to read the code first, then repair the fault, and clear the record after the repair.

For safety-system warnings such as airbag or ABS, see a workshop before clearing; the decision to clear is yours. Every clear made with 2dakika is recorded; you can find the conditions, and how the record appears in the report, on the page about clearing fault codes.

What does the 2dakika report show when the light comes on?

Plug the 2dakika OBD-X1 into the OBD port, usually under the steering wheel, turn the ignition on (without starting the engine) and start the scan in the app. On most cars a scan takes under 2 minutes. The device reads the modules it can reach in the car; the report shows these records in plain language:

• Current faults and fault history, in separate lists. Over 50,000 fault descriptions are matched to the car's make.
• Each code's severity: Priority, Warning or Info.
• Where the engine computer stored them, the conditions at the moment the fault occurred, such as engine temperature, rpm and speed.
• On cars that report it, the distance driven since the fault memory was last cleared, and the distance driven with the warning light on.

Severity is a guide

Severity shows how much attention a code generally needs; it doesn't assess the car's current condition in place of a mechanic. If the warning light is flashing or there are symptoms while driving, have the car checked at a workshop, whatever the severity.

The check engine light mostly comes on for engine and emissions-related faults; systems such as ABS and airbags have their own warning lights. The report brings together the faults in the modules the device can reach. The modules and information that can be read vary by car: 2dakika works with cars and light commercial vehicles that use K-line or CAN; trucks, buses and Tesla are not supported. If some modules can't be reached, that doesn't mean the car is faulty.