You notice the yellow check-engine warning light on the instrument panel turns on and stays on when the engine is running but you don’t feel anything is wrong. You assume that the problem is with the wiring and ignore it. If you encounter this problem you don’t have to immidiately pull to the side of the road or even go to the workshop but you shouldn’t disregard this problem.
On any modern car, the onboard computer system is based on a central computer called ECU (Engine Control Unit) or EMS (Engine Management System). The ECU receives information about the engine condition and other parts of the car from a system of sensors.
The information from the sensors is read by the ECU and compared to the ECU’s memory and changes could be made to the way the engine is running. If for example when the car is under heavy load, the ECU will made adjustment so more fuel is fed to the fuel injector and the changes will be made to the engine timing.
The car system works perfectly provided all the sensors, wiring and ECU are in good condition. The check-engine warning light will turn ON when one or more sensors provides reading that don’t make sense to the ECU and the ECU will ignore the reading.
You still could drive a car with the warning light ON but make sure you send the car to the mechanic ASAP. If you ignore this symtom, the consequences could be costly and the fuel consumption could be higher too.
Before you send the car to the mechanic, what you can do is check for any disconnected hoses (vacuum hose) or wiring under the hood. Check also the air filter whether it is clocked or not. If you have spare air filter, replace the old one and check if the warning light stays off or not.
The next best thing to do is to send the car to the authorised mechanic of the car manufacturer. Usually they have better equipment to check what is wrong to the ECU, the fault codes, and also their mechanic are fully trained. Unlike the oil pressure and temperature warning light, the check-engine warning light doesn’t warn you the specific problem. After the mechanic obtained the fault code, he/she could narrow down the problem and could estimate what will be the cost of the repair.
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I have gone thru this light issue with four different vehicles, 2 Land Rovers, one Lexus and my Porsche 911. So far here is what I have found. (And I will say that my Brother is a Mechanic at an American Car Company, but he has always worked on my non-American cars.) He says, as well as the Mech’s that I have spoken to at the various dealers.
1- Like you said it could be any number of Factors, and if the car is in the US, the ECU will usually be based on the emissions system. (For Example; on the Lexus the light came on just because the Gas Cap wasn’t on tight enough!)
2- It could also be factors such as too heavy of a load, or the Weather may be too cold.
3- In my Land Rovers, the light is just known as one of the Big three, as it is pretty much always on, especially after the vehicle turns about 4 0r 5 years old. I have had it on my oldest Rover, and I chk the codes often, and nothing ever comes up.
4- Lastly on my Porsche 911, when it came on, I like many other people freaked, the last thing I wanted to do was spend $15k (on the low side) for an engine. So I pulled over and checked the main things in the engine and I found that some Chipmunks had went up into the engine compartment and made a toasty home after I came in from a brief drive the other day. And yes they got a hold of some air hoses; hence the ECU light.
Basically, I agree with everything you say, I just wanted to share the scope of “Non-issues” I have had with this ECU light. Lastly my brother pretty much said it best when he said that if the light comes on, it is just to bring to your attention that there may be an issue. If it stays on, find out what the problem is, ie..go to a mechanic, however if the light starts blinking…….Stop the car right there turn it off and call a tow truck driver, because you have just entered a potentially serious Zone.