One might say the most challenging part of being an automotive service technician is diagnosing a problem before it can be fixed. Cars are made up of a bunch of complex systems. There usually could be a number of reasons for any given symptom. So it’s challenging to track down the actual cause of the problem. And it can be frustrating for the vehicle owner because it can take time and money to get to the bottom of a problem. If it’s not something obvious, it’s easy for the customer to focus on the fixing and not the diagnosing.

Let us introduce you to something we’ll call Customer Detective Work – that is helping the technician find clues to what’s wrong.

We start with the detective basics: What, Where and When. Play along with me. …You come in and your car is making a funny sound…

Q: Where’s the sound?
A: Around the right front wheel.

Q: What kind of sound?
A: Kind of a clunk, clunk sound.

Q: When do you hear the sound?
A: When I turn and accelerate.

Q: Right and left? Forwards and back?…

Do you see where we’re going? You’re gathering additional information to help the technician know where to start. Based on your car and the tech’s experience, he’ll know where to look and can start with the obvious suspects.

You can see how that would be more helpful than dropping the car off with a note that says “making a funny noise”.

When you think you need to bring a vehicle in, make some notes about the problem. Rather than just saying “it’s leaking” tell the tech the color of the fluid, and approximately where under the car you see the puddle.

Things like the car is stalling or sputtering are often very hard to diagnose because they’re intermittent. They may not happen every time you drive and usually aren’t happening when you actually bring the car in. So, it is a big help for you to describe what’s happening in as much detail as possible.

Your auto technician will need to be able to duplicate the problem if possible so he needs to know details, like it stalls after it’s been driven for about 20 minutes and I go over 50 miles an hour.

If the tech can experience the problem personally, he’s better able to make a diagnosis and repair. And, then test to see if the repair solved the problem.

Today we’re going to talk about on-board diagnostics and the questions we hear from viewers who need answers about diagnostic services. They want to know what diagnostics are, what’s involved and what the benefits are. They really want to understand the value of diagnostic scans by a trained technician.

These are valid concerns. If you don’t understand something it’s really hard to know its value. Let’s start with some history.

Since 1996, all cars and light trucks have been required to use a standardized diagnostic system to help repair technicians determine what’s wrong with your vehicle. The diagnostic system works with the vehicle’s Engine Control Module – the computer that controls many engine functions.

The computer monitors dozens of components and processes. Depending on what the sensors read, the computer will make adjustments to compensate for conditions and minor problems. When there is a condition that it can’t adjust for, the computer will turn on the check engine light.

It is also called the “service engine soon” light on some vehicles. The warning light signals you to get into a service center so that the trouble code can be read and the problem can be fixed. Your service center will have a scan tool and powerful software that will help the technician diagnose the problem.

If you’ve searched for check engine light on the internet, you may have seen that you can buy an inexpensive scanner or go to an auto parts store to have the trouble code read to tell you exactly what’s wrong.

That’s a common myth. The code itself doesn’t tell you what’s broken. It starts you looking in the right place. It tells you what engine parameter is out of range – but it won’t tell you what’s wrong or how to fix it.

Let’s say you think your daughter has a fever. You take her temperature and it reads one 102 degrees. You’ve confirmed a fever, but you don’t know what’s causing it. Is it a 24 hour flu, an infection, appendicitis or leukemia? A fever is a symptom of all of these medical problems, but it takes a skilled physician’s examination and additional diagnostic tests to find out what is actually causing the fever.

An example of a trouble code could be: P0133, which reads “Bank 1 sensor 1 circuit slow response”. This means that the front oxygen sensor has a slow response time to changes in the air-fuel mix. If that’s all you knew about cars, you would think your oxygen sensor was broken and would replace it. Now, it could be the oxygen sensor – but it could also be a bad or contaminated airflow sensor, exhaust leak, electrical problem, an intake manifold leak or any of a number of other things.

You can imagine a lot of oxygen sensors have been replaced because of that code. So the on-board diagnostics point the way to where the trouble lies, but it takes some skill and high-tech equipment to actually pinpoint the problem. The cheap scan tools that a consumer can buy do not have the ability to retrieve some of the operating history that’s stored in the engine control computer. That history’s very helpful in diagnosing the problem. Service centers invest a lot of money in high-end diagnostic tools to help solve the mystery and get you back on the road as soon as possible without replacing a lot of parts that don’t need replacing.

So, on-board diagnostics provide a powerful starting place for a highly-trained, well-equipped technician to get to the bottom of your problem. When your check engine light comes on, get it checked. If the light burns steady – don’t panic. Get in soon to have the engine scanned. A flashing check engine light means that there is a severe engine problem. Get in as soon as you can – waiting too long can lead to very expensive damage. And try to not drive at high speed or tow or haul heavy loads with a flashing check engine light.

See more automotive videos on our Auto Video Tips library.

Did you know that today’s cars carry more computer power than the Apollo 121 Lunar Module that landed on the moon in 1969? New cars have as many as 12 networked computers and over five miles (eight kilometers) of wiring.  In fact, for the last decade or so, computers have been controlling about 85% of your vehicle’s functions.

Cars have sensors for manifold air temperature, coolant temperature, manifold air pressure, airflow, throttle position, vehicle speed and oxygen content. All of this electronic wizardry is pretty complicated.  So how do you know when there is a problem?

It’s simple – the Check Engine light comes on.  The computer monitors all the sensors and uses that information to decide what to adjust such as the fuel mix, spark timing and idle speed. In addition, the computer monitors its own circuits.  When it finds a fault, it turns on the Check Engine light and stores a trouble code in the computer.

It can be pretty disturbing when the Check Engine light comes on.  We wonder just how urgent it is. Generally speaking, it is not critical like a temperature or oil pressure light.  When you get one of those it means STOP NOW!  When the Check Engine light shows up, you should get to your service center to find out what the matter is as soon as possible.

Since 1996, there has been a strong emissions control component to the Check Engine diagnostic. But if your Check Engine light flashes on and off, you know that it is more urgent and you need to get it checked immediately to prevent damage.  You should slow down and avoid towing or heavy loads until you can get it checked out.

Your technician has special diagnostic equipment that will retrieve the trouble code from the computer and help him determine what is wrong.  From there, he can fix it and get you back on the road.