A malfunction indicator lamp (MIL), commonly referred to as the "Check Engine Light", is a tell-tale

Tell-tale (automotive)

A tell-tale is an indicator of the status or malfunction of a system within a motor vehicle. Tell-tales consist of a light bulb or an LED which lights up a symbol or text legend...

 to indicate malfunction of a computerized engine management system. It is found on the instrument panel of most automobiles. When illuminated, it is typically either an amber or red color. On vehicles equipped with OBD-II

On-board diagnostics

On-Board Diagnostics, or OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or a repair technician access to state of health information for various vehicle sub-systems...

, the light has two stages: steady (indicating a minor fault such as a loose gas cap or failing oxygen sensor

Oxygen sensor

An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...

) and flashing (indicating a severe fault, that will eventually destroy the catalytic converter, such as a misfire). When the MIL is lit, the engine control unit

Engine control unit

An engine control unit is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running...

 stores a fault code related to the malfunction, which can be retrieved with a scan tool and used for further diagnosis. The malfunction indicator lamp usually bears the legend , , or a pictogram of an engine. In the United States, specific functions are required of the MIL by EPA

United States Environmental Protection Agency

The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 regulations.

The MIL appeared in the early 80s along with computerized engine controls. Even the earliest systems, such as GM's CCC (Computer Command Control) system had self diagnosis functionality. When the computer detected a fault, it illuminated the MIL. Up until OBDII, on most cars the MIL could output codes, when two pins on the ALDL are jumped, the light would flash the codes, for instance (blink) (pause) (blink) (blink) for code 12. Some manufacturers, such as Honda, retained this feature even after OBDII.

"Trouble" indicator

Some older vehicles had a single indicator labeled "trouble" or "engine"; this was not a MIL, but an "idiot light"

Tell-tale (automotive)

A tell-tale is an indicator of the status or malfunction of a system within a motor vehicle. Tell-tales consist of a light bulb or an LED which lights up a symbol or text legend...

 meant to indicate serious trouble with the engine (low oil pressure, overheating, or charging system problems) and an imminent breakdown. This usage of the "engine" light was discontinued in the mid-1980s, to prevent confusion with the MIL. The MIL is commonly referred to today as the "check engine light" or the "service engine soon light".

Odometer triggering

Some vehicles made in the late 80s and early-to-mid 90s have a MIL that illuminates based on the odometer reading, regardless of what is going on in the engine. For example, in several Mazda

Mazda

is a Japanese automotive manufacturer based in Fuchū, Aki District, Hiroshima Prefecture, Japan.In 2007, Mazda produced almost 1.3 million vehicles for global sales...

 models, the light will come on at 80,000 miles and remain lit without generating a computer trouble code. Volvo had a light labeled "Lambda", lambda sond being another name for oxygen sensor. This was done in order to remind the driver to change the oxygen sensor.

Some American-built 1973–1976 Chrysler Corporation vehicles had a similar odometer-triggered reminder: "Check EGR", which was reset after service at a Chrysler dealership.
The MIL is also illuminated prior to starting the engine, along with other tell-tales on the dashboard, to demonstrate that the lamp is working and not burned out. The lamp will turn off once the car is started if no monitored faults exist. The MIL will also illuminate if the engine has been cranked but failed to start after returning the ignition to "ON." In most modern vehicles this is usually due to not giving the engine enough cranking time to start. In pre-fuel injection years many cranking attempts were sometimes required to start the car. If the engine fails to start one the second or third attempt then an actual visual check of the engine could be required. If the engine suddenly stalls or is triggered by an overload, such as on a stick shift car, the MIL will also illuminate until the engine is started again.

False triggering

Higher amounts of methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

/ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 (or other additives) than the engine is equipped to handle may trigger the MIL(see E85

E85

E85 is an abbreviation for an ethanol fuel blend of up to 85% denatured ethanol fuel and gasoline or other hydrocarbon by volume. E85 is commonly used by flex-fuel vehicles in the US, Canada, and Europe. Some of the benefits of E85 over conventional gasoline powered vehicles include the potential...

). These burn differently from gasoline, and the EFI system may mistakenly interpret the oxygen sensor's readings as being incorrect. Oxygenated gasoline can cause this false triggering in early EFI systems.