Mechanically Speaking: Revisiting the popular ‘Check Engine’ light dilema

"We auto mechanics all have customers that continue to drive with that (check engine) light on."

Sometimes I feel like I have beat this subject to death but the subject comes up again and again. That little orange light! We auto mechanics all have customers that continue to drive with that light on. Some have resorted to a piece of black tape. Maybe others have resorted to a bulbectomy (removed the bulb).

That little orange light, the one that is a picture of an engine, or the words check engine, or the Greek symbol for lambda. That is the one I am talking about. When that light stays on or turns on when you are driving there is a problem with the way your vehicle is running. It is no longer running optimally. It needs to be fixed!

If the light is not flashing while driving and your vehicle seems to be performing okay you do not have to stop driving the vehicle or bring it directly to your mechanic but it is time to set up an appointment. That light may be the only clue that something is up but it is a clue. Your vehicle may still start and run flawlessly in your opinion and the problem may be trivial. Unfortunately the same light that is signally a trivial problem is also used to signal a more serious problem. How do you know when a problem has changed from trivial to serious? You don’t.

That light can turn on for virtually hundreds of reasons. Once you have decided to ignore it, how do you know when one trivial reason has turned into one trivial reason and one serious malfunction? See what I mean?

Some might say light on or off the vehicle runs the same. Yes, that might be true. Unfortunately or fortunately modern day computerized electronic control systems can run your car relatively smoothly when all is not well.

Auto manufacturers seek to meet fuel economy targets, performance targets, and emission targets with as inexpensive control systems as possible. The gains each year are at the level of single percentage points or even fractions of a percentage point.

One example. In the early nineties most port fuel injected vehicles (one fuel injector for each cylinder) injected fuel once per engine revolution. All the injectors were opened at the same time. For all intents and purposes these vehicles ran perfectly. The late nineties ushered in sequential port fuel injection. Now each injector would be turned on just before the cylinder it feeds was about to suck in the air and fuel it needed. At light loads this method saved some fuel and reduced emissions and produced more power. Implementation of sequential fuel injection required a few more sensors and some software work. The manufacturers felt it was worthwhile.

One of the key sensors to sequential fuel injection on most vehicles is a camshaft position sensor. It is used combined with a crankshaft position sensor to exactly determine which cylinder is sucking in air and when. The fuel from the injector for that cylinder can be injected right when it is needed.

When that camshaft position sensor fails your engine won’t quit running. It likely will not skip a beat. Instead of sequential injection your engine will switch to batch firing (all injectors open at the same time). Efficiency will go down, emissions will go up. A malfunction code will set in the engine computer system. The check engine light will be signaled to turn on.

The light is already on for some more trivial problem. Now you have two problems without even knowing it. The second one is wasting some of your money. Wouldn’t you like to know?

Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC. E-mail:

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