![Vw engine check](https://upload.wikimedia.org/wikipedia/commons/c/c9/Vw_engine_check.jpg)
A check engine light or malfunction indicator lamp (MIL), is a
tell-tale that a computerized engine-management system uses to indicate a malfunction. Found on the instrument panel of most automobiles, it usually bears the legend , , , , , or a pictogram of an engine—and when illuminated, it is typically an amber or red color.
The light generally has two stages: steady (indicating a minor fault) and flashing (indicating a severe fault). When the MIL is lit, the
engine control unit
An engine control unit (ECU), also commonly called an engine control module (ECM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by re ...
stores a fault code related to the malfunction, which can be retrieved—although in many models this requires the use of a
scan tool. This warning light can indicate almost anything from a loose gas cap to a serious knock in the engine.
In the United States, specific functions are required of the MIL by
EPA
The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
regulations.
History
Some older vehicles had a single indicator labeled "trouble" or "engine"; this was not an MIL, but an "
idiot light
A tell-tale, sometimes called an idiot light or warning light, is an indicator of malfunction or operation of a system, indicated by a binary (on/off) illuminated light, symbol or text legend.
The "idiot light" terminology arises from popular f ...
" 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 appeared in the early 80s along with computerized engine controls. Even the earliest systems, such as
General Motors
The General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. It is the largest automaker in the United States and ...
' CCC (computer command control) system had self diagnosis functionality. When the computer detected a fault, it illuminated the MIL. Up until
OBDII
On-board diagnostics (OBD) is a term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle sub-systems. The amount of diagnostic inf ...
, 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.
The MIL is commonly referred to today as the "check engine light" or the "service engine soon light".
Odometer triggering
Some vehicles manufactured from the late 1990s to mid 2000s have a MIL that illuminates based on the odometer reading, regardless of engine operation. For example, in several
Mazda
, commonly referred to as simply Mazda, is a Japanese multinational automotive manufacturer headquartered in Fuchū, Hiroshima, Japan.
In 2015, Mazda produced 1.5 million vehicles for global sales, the majority of which (nearly one m ...
models, the light will come on at and remain lit without generating a computer trouble code. Volvo had a light labeled "lambda", ''lambda sond'' being another name for
oxygen sensor
An oxygen sensor (or lambda sensor, where lambda refers to air–fuel equivalence ratio, usually denoted by λ) or probe or sond, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analysed.
It was ...
. This was done in order to remind the driver to change the oxygen sensor. Some American-built 1973–1976
Chrysler Corporation
Stellantis North America (officially FCA US and formerly Chrysler ()) is one of the " Big Three" automobile manufacturers in the United States, headquartered in Auburn Hills, Michigan. It is the American subsidiary of the multinational automoti ...
vehicles had a similar odometer-triggered reminder: "Check EGR", which was reset after service at a Chrysler dealership.
Other triggering
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 on 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
manual transmission
A manual transmission (MT), also known as manual gearbox, standard transmission (in Canada, the United Kingdom, and the United States), or stick shift (in the United States), is a multi-speed motor vehicle transmission (mechanics), transmission ...
car, the MIL will also illuminate until the engine is started again.
False triggering
Higher amounts of
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
/
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
(or other additives) than the engine is equipped to handle may trigger the MIL (see
E85
E85 is an abbreviation typically referring to an ethanol fuel blend of 85% ethanol fuel and 15% gasoline or other hydrocarbon by volume.
In the United States, the exact ratio of fuel ethanol to hydrocarbon may vary according to ASTM 5798 that ...
). 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.
See also
*
OBD-II PIDs
OBD-II PIDs (On-board diagnostics Parameter IDs) are codes used to request data from a vehicle, used as a diagnostic tool.
SAE standard J1979 defines many OBD-II PIDs. All on-road vehicles and trucks sold in North America are required to support a ...
("Parameter IDs")
References
{{Car-interior
Vehicle parts