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February 22, 2012 - Larry DeHays
All cars have one. Lots of cars drive around with it turned on. It tells you to do something that never makes it stop. It might go off on the way to the repair shop, or come on as you leave the shop. We could be talking about your back seat driver's running commentary, but we're not. We're talking about something from the twilight zone. It's called the "check engine" light. Why do we have one, and what does it mean?

We have one because congress mandated it. What can you expect when a bunch of lawyers get involved? They said that cars had to tell you if they began to polute the air. That's what the light means, something has happened which has raised the emission level from the car. It doesn't mean you are low on oil or water, or anything else you can reasonably check yourself, so don't ask me why it says to check your engine. There are dozens of sensors all over the car which are monitored by a computer system. If one of those sensors detects a reading out of the normal range, it causes the "check engine" light to come on, even if it has little to do with the engine. For instance there is a sensor which measures the vapor pressure in your fuel tank, which is controlled to avoid letting gas fumes escape, so if the gas cap is left loose, the "check engine" will come on. You'll do a lot of engine checking before you discover that loose cap.

The point here is that there is only one light for multiple problems, instead of different lights for each problem. The light does not identify the problem, but the car's computer does. It requires a special scanning tool to interrogate the computer and a trained technician to diagnose the result. There is no such thing as a machine which you can plug in and have it tell you what part to replace, regardless of what you have heard. An xray machine doesn't tell you what drugs to take. These are tools to be used by diagnosticians who understand the systems involved.

Some of the problems (like the gas tank vapor monitor) will not affect the way the car runs, or cause you to break down, but some of them might. In Florida there is no law requiring you to fix an emissions problem if you don't want to. In states with vehicle inspections you have to get them fixed, so count your blessings if you live here in "paradise", but it could be a mistake to ignore the light. For instance, if a problem is making the gas mixture too rich, and you ignore it, that rich mixture could lead to one or more damaged catalytic converters in the exhaust, which will have to be replaced when they plug up. That can cost big bucks. The rich mixture might have been fixed cheaply. Some people try to bypass some of the emission system, like removing the catalytic converters, but this results in the light staying on forever. You can't out smart it, but you can ignore it, or cover it with tape. Same thing with your back seat driver.


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