A Proper Investigation Of The Check Engine Light

The offending article: Investigating the Dreaded 'Check Engine' Alert

I have an issue with any car article in which the very first piece of advice is to replace a ~200 dollar part. There are tests you can do to isolate the problem, which is why you take your car to a good technician or you learn how to diagnose it yourself.

Here is my guide to the Check Engine Light for 1996 cars and newer:

What is the Check Engine Light?
The Check Engine Light (or MIL, Malfunction Indicator Lamp) is an orange, yellow, or red light on your dash, that usually has some kind of picture of an engine, engine and text, or just text. These are not to be mistaken for the maintenance lights also commonly found on newer cars.

When does the MIL come on?
The MIL illuminates when the computer detects a problem with the car that would cause its emissions (polluting exhaust gases) to exceed 1.5 times federal standards (usually a type B code), or when there is a severe-enough issue with fuel management or misfires (type A codes).

When doesn't the MIL come on?
Obviously, the reverse is true, but just to reiterate: the MIL does not generally come on for type C and D codes. These codes usually apply to things like burnt out lightbulbs, climate control faults, seat heater issues, etc. It may also not come on for what's called a driveability issue that doesn't cause emissions to exceed 1.5 times federal standards; driveability issues are things that drivers complain about, like shaky steering wheels, stumbling on acceleration, overheating, and so forth. What this means is that you might notice a problem with the car's performance, like reduced gas mileage, but the light may not illuminate.

What does the MIL tell you?
The light itself only tells the technician that there is some kind of fault with the emission control systems on the vehicle. In order to get more information from the computer, we must plug a device called a scanner into a port on the car (usually under the steering wheel or near the glovebox). The scanner will allow us to pull what is called a code (remember the types A-D earlier?). The code does not tell you exactly what it wrong with the car; it only gives you a compass and a map. The tech must find the location herself.

This is important to remember. If your car has a misfire (in which no explosion is occurring inside the engine), the computer cannot tell you if it's because of a vacuum leak, low fuel pressure, lack of air flow, or issues with the electrical system (among other possible causes). If the tech is lucky, the computer may distinguish between LEAN MISFIRE CYLINDER 1, in which case the tech can then test for vacuum leaks or fuel pressure issues on cylinder #1, but rarely is the computer this helpful. It's similar to getting an error on Windows XP that gives you the file number 000x000.111-3223, but gives you no info on why the error occurred.

Is it an emergency if my MIL comes on?
Maybe. Having the check engine light on itself may not affect the car's performance noticeably, but a badly performing car will impact your wallet, by reducing gas mileage so you pay more for fuel more often and by paying more later on as the issue causes other problems, creating larger repair bills. You absolutely need to worry if the MIL is flashing; pull over to the side of the road and call a tow truck, as you either already need or will definitely need a new catalytic converter for your car - a $200+ part.

What are some common causes for a lit MIL?
Many things can affect the MIL, which is why I hesitate to give a "quick" guide. A scanner is really your best place to start, and different codes have different common causes. However, from what I've seen, most MILs tend to fall in one of three places:

1) Gas cap - A loose, damaged, or old gas cap can cause the car to fail EVAP, a self-test the car performs to ensure that gas fumes aren't escaping from the vehicle. Faulty gas caps can usually be detected by the odor. A good guideline is to listen for two or three clicks when tightening the gas cap after fueling.

2) Sensor issue - This issue occurs when the computer sees a reading from sensor that either doesn't make sense (rationality), opens (continuity), or shorts (functionality). Rationality only applies to post-1996 cars; pre-1996 only detected opens or shorts. A good example of a rationality failure is when the computer detects that the RPM has gone up, but the Throttle Position Sensor (TPS) says the pedal hasn't moved.

3) Misfire - For repetition's sake, the lack of explosion inside the engine is called a misfire. Misfires can have many causes, but usually results from a fault in one of four basic systems: fuel, air, spark, compression. Unfortunately, pulling a code from the computer rarely helps you detect which system is at fault, so this is where diagnostic skills and following a good test procedure really come into play. I tend to pull the code, then go by checking the easy, visible stuff first (like loose spark plug wires). This helps save time and headaches later on down the line.

A note about scanners and sensors:
One of the most helpful diagnostic tools available to a technician is the live data stream accessed through the scanner. This allows the technician to view what the sensors are reading while the car is running, so the tech can view the problem "live", if the problem can be replicated. There is a very large caveat with this data stream: the computer will sometimes substitute a "good" value when it detects an issue with a sensor. Doesn't that sound annoying? The code will tell the technician that there is a problem with the O2 sensor, but when you look at the data the O2 sensor is putting out, it looks fine. Finding a savvy technician who is equipped, both mentally and literally, is a necessity.

How long does it take for the MIL to go out?
However many hours the bulb is designed to work for, if the problem isn't fixed. You might look at me funny, but it is a common story among techs for customers to show up and wonder what happened because their MIL went out - after having been on for as long as two years.

If the problem is repaired, there are two options. The technician can clear the code from the computer, essentially like hitting the reset button. The issue here is that all the car's self-tests (called monitors) are also reset, so before getting a smog test nearly all the monitors must be completed and passed or the car will fail smog. The second method is by driving the car. The MIL will clear after the previously malfunctioning component passes its test 40 times (the requirements for the test depend on the component, but can be as often as once per drive, or may require certain driving conditions be met, like sustained speed, engine temp, or fuel tank level).

What about older cars?
Hope you still have the owner's manual. In 1996, the EPA established OBDII, which standardized the information required of the manufacturers for all vehicles, allowing techs to easily access information with one tool. OBD, or OBDI, covers older cars, but there were no standards, so each manufacturer played by its own rules. Flash codes were common, which were codes retrieved by counting the times the MIL flashed. Sometimes you could connect a scanner to the car, but other times you had to use a complicated system of jumper wires and paperclips to be able to retrieve the code. Best course of action: take it to a tech, or research it yourself.

Note: I take no responsibility for the accuracy of this article, especially since the automotive industry is constantly evolving. If you're a tech and you find my info inaccurate, point me to a source and I'll make the appropriate correction.