The Infamous Check Engine Light

Rodney Cropper with Good Ole Days Auto Service

So your car’s “check engine” light comes on. Now what? The reason the check engine light comes on, and what needs to be done to properly diagnose it, is oftentimes misunderstood.

The check engine light lights up when the computer that monitors engine and transmission performance (Powertrain Control Module or PCM) has detected an input sensor reading (such as coolant temperature) or an output component that it actuates (such as a fuel injector) is out of its pre-programmed window of normal operation for a given condition. Got that?

In other words the PCM doesn’t like what it is seeing. The PCM will then store what is called a trouble code in its memory and turn the check engine light on. A technician will retrieve these trouble code(s) from the computer’s memory with a scan tool. This is usually where the misinformation starts.

A common belief is that a repair shop will have a computer that you “hook up” to your own vehicle and it will tell you exactly what to replace. Do-it-yourselfers tend to believe if they take their car to an auto parts store, technicians will read the trouble code for free, and the trouble code will then tell them what to replace. This is not entirely the way it works.

Let‘s take an oxygen sensor lean trouble code for example.

• Could the oxygen sensor be faulty?
• Could it also be that there is a problem somewhere else causing the engine not to receive enough fuel?

Although it could be either one of these, the much more likely scenario is going to be B. The first thing that has to be done is to determine if the engine is, in fact, not getting enough fuel. Most of the time with an oxygen sensor lean code, we find that the sensor is reading correctly and the engine is not getting enough fuel. In that case, the oxygen sensor trouble code has alerted you to the fact that the engine is not getting enough fuel, not that the sensor is defective. You would then start looking at the scan tool data and checking other sensor readings. Sensors can be off enough to cause problems, but still pass the PCM’s test parameters. The technician uses their training to determine if those readings could cause the vehicle condition being displayed.

If all the sensor readings look good, then we would need to start looking at other issues that could cause the engine to not get enough fuel:

• Is there an injector not spraying enough fuel due to either a nozzle restriction or an internal electrical failure?
• Is there a vacuum leak somewhere?
• Is the fuel pressure correct?

The only way to find out is to check these and many other things that could cause the vehicle to not get enough fuel. We start with the most common things that experience tells us would likely be the cause. It can be a very time consuming process, but is necessary for an accurate diagnosis. Replacing parts that aren’t needed and that don’t fix the problem can get very costly.

Beware of “free” diagnostic services. Places that offer free diagnostics usually haven’t invested in a good scan tool and oscilloscope that can cost thousands of dollars each, both of which are needed if you are serious about computer system diagnostics. Most “free” diagnostics involve hooking up a code reader that can be bought for less than $100 and telling you to replace the sensor or component that the code is referring too. As we have learned, without performing any tests that is just a guess and not a diagnosis.

Just ask yourself this question: Would you spend thousands of dollars on test equipment, take continuing educational classes to keep up with technology, spend (on average) over an hour of your time diagnosing a complex computer system problem and then not charge anything? Doesn’t really make much sense does it?

Another question about the check engine light frequently heard is, “The car is running fine. Do I really need to have it checked out?” My recommendation would be to have it checked out. If you wait to have it checked out, you may end up replacing several components by the time it gets bad enough to cause the car to run poorly. Depending on the problem, you can also cause damage to other components if it’s left unchecked for any length of time. It’s best to “check up” on a “check engine” light designed to alert a car owner to a problem. Better safe than sorry.

Happy motoring!

Do you have a question on how to take better care of your car? Email your questions to Rodney Cropper at goodoledaysauto@aol.com.