CAR CLINIC | Why you shouldn't ignore your ‘check engine’ light

One of the scenarios drivers of modern cars dread is a bright, yellow warning light showing the outline of an engine block, which suddenly, for no apparent rhyme or reason, switches on in the instrument cluster. We are talking of the ominous “Check Engine” light, also known as a MIL (Malfunction Indicator Light).

What do you do, and how much time do you have to do it? That is precisely the problem with this particular warning light — you are told, usually at the most inopportune time — that your car has a problem, but you have no idea what it is or how close to a breakdown you may be.

A driver's biggest challenge in such a situation is to stay cool, calm and collected. Try to remember for future reference if there were any unusual noises or events immediately before the MIL lit up, and scan the instruments on the dashboard for any perturbing readings that might explain the computer's consternation. It might also be helpful to keep the following points in mind.

A “Check Engine” light can come on for hundreds of different reasons, from trivialities like a loose fuel filler cap (remember, the On-Board Diagnostic system originated with a concern for hydrocarbon emissions) to really serious emergencies like low oil pressure due to the engine oil escaping through a crack in the sump sustained at a pothole. Alarming as it may be, especially if it's the first time you see this light illuminated on a normally very reliable car, it is reassuring that serious problems will almost always be accompanied by clear warning signs — disconcerting noises, erratic handling, loss of power, and so on If nothing like this is noticed, it's probably safe to drive cautiously to the nearest shelter.

Sometimes one or more of the item-specific warning lights will switch on together with the MIL. The following three warning lights should always be investigated as a matter of urgency, whether they trigger the “Check Engine” light or not:

— The low oil pressure light (red oil squirt can)

— The overheating light (red thermometer in wavy water)

— The brake warning light (red brake disc between clamps)

The following two warnings also call for prompt attention:

— The charging system light (red battery outline with + and – markings)

— The low tyre pressure light, if fitted (yellow cross section of casing with exclamation mark)

On some vehicles the “Check Engine” light will be flashing when the diagnostic system considers the fault serious enough. This is meant to warn the driver that he should think twice before proceeding. It will be wise to heed the warning.

The powertrain control module is programmed to put the engine and transmission in so-called “limp home” mode when information received from sensors indicates a malfunction that could result in serious damage to components of the powertrain. In this mode the engine power will be drastically curtailed, the rev limit will be reduced, usually to about 3,000rpm, and if the car has automatic transmission, it will not shift higher than third gear. And the “Check Engine” light will always light up. One possible cause of “limp home” mode is overboost on turbocharged engines. The restrictions are intended as a safety feature to protect the powertrain by significantly lowering the stresses in the engine and transmission, while still allowing the driver to crawl homeward.

Unnerving and mystifying as the MIL might be, it has one redeeming feature — when it switches on, one or more explanatory fault codes are logged on the engine computer. These can be retrieved with an OBD scan tool. But even before that is done, the owner should inspect the engine compartment and the drivetrain very carefully under a good light. It may just be something as simple as a loose wire to a sensor, or a slack-fitting connector allowing water penetration, or a low fluid level, say in the brake fluid reservoir — something that you can easily rectify yourself, thereby saving a tidy sum of money.