A leading character in a well-known television comedy is frequently nagged by her nemesis to do something about her car's check engine light, which is permanently lit.
Is she wrong to ignore the light? Almost certainly. Even if the check engine light itself is faulty, how will she become aware of a genuine engine fault eventuating if she persistently ignores the warning light?
Nearly every operational function of the internal-combustion engine in a modern motor car is monitored by computer.
Yet apart from the evidence of your own senses (vision, hearing and smell) the only way to know if your car's engine is in trouble is a little warning light in the instrument cluster. By the time you note the steam escaping from under the bonnet, the loss of power or the unexpected rattling noises it's probably too late. The bill could cost thousands and the inconvenience could be incalculable.
While the check engine light is not especially informative, it does provide the driver some early warning of a problem – hopefully before the problem leads to an expensive fix.
When lit the check engine light may indicate a fuel problem, an overheating problem, an ignition problem or anything else that could pose a serious concern for the motorist. The engine may be low on lubricant or coolant, it may be trying to combust an unrecommended fuel, or there may be a short in the electrical system somewhere.
Essentially, anything the engine management system ECU (Electronic Control Unit) – also known as the powertrain control module – can't fix for itself is reported as an engine fault via the OBD (On-Board Diagnostic) system. This is typically when the check engine light will illuminate. It may be evidence of poor maintenance or it could due to be an unforeseeable circumstance (filling a petrol tank with diesel by mistake, for instance).
And while the check engine light may be an automated response to something quite benign, don't trust to luck. Baby the car – accelerate gently, try to maintain a low, steady speed – and pull over at the nearest convenient service and repair workshop to have the check engine light checked.