What is Diagnostics FAQ

Diagnostics.

What I hope to do on this page is to give a clear and simple explanation of what is involved in “diagnostics”.

I find that a high percentage of customers have very little understanding of what “diagnostics” are. Because it is a very technical

and complicated thing it is understandable that the majority of customers don’t know what is involved. Sometimes this leads to

unrealistic expectations when a diagnosis becomes involved. Diagnostics is a relatively new field which is constantly changing and evolving, as such, it is understandable that there is a lot of confusion as to what it actually is. This can lead to disappointment when a diagnosis is not instant.

A diagnostic computer, when plugged into a car, reads information from the cars computer. This information is in the form of fault codes and other data. Sometimes a fault code can give an instant diagnosis, but more often it just gives a direction for further diagnosis. In other words, a computer does not tell you what is wrong. For the information that is available from the computer to be useful it must be combined with information from yet another computer, the one between the mechanics ears. The information available from the computer is only useful if the person interpreting it understands the data. Without the knowledge to interpret the information from the diagnostic computer the mechanic would be unable to make an informed decision as to how to proceed with a repair. This is why expensive mistakes happen when a diagnosis is performed based only on the information from the computer. The computer will often point to an expensive component as being faulty but it is up to the man working on the car to test this before replacing it. There are a whole range of tools available to today’s mechanics to enable them test various parts..The diagnostic computer is just one of these tools. To do an accurate diagnosis a mechanic needs the correct tools, a very high level of knowledge and access to technical information about the electronic system he is working on. If he is lacking even one of these three he will struggle.

Diagnostics on modern cars is a complex undertaking that requires considerable investment in both equipment and knowledge. Due to this, over recent years diagnostics has developed into a very specialised business.

Hopefully the above will help to clarify this subject somewhat. For anyone reading this who wants slightly more in depth information I have structured some FAQ's below.

I would welcome any questions or comments by email on this article and would be happy to add to it for the sake of clarity.

FAQ's

Do you have a computer that fixes cars?

This is a question I am often asked. The accurate answer is no, because computers do not fix cars. Most people’s perception of diagnostics is a computer that plugs into a car and fixes it, but in reality it is a lot more complicated than that. The computer is just one tool out of a range of available tools that are needed to diagnose car problems.

What can the diagnostic computer do?

A lot depends on the computer. Some are more capable than others. Simple OBD or EOBD readers costing a few Euros can do very little, expensive dealer tools can do a lot more and there is a huge range in between. The main thing to remember is that every diagnostic computer is limited by the abilities of the computer it is talking to. This second, often forgotten computer, is a lot more important that the one the mechanic is holding in his hands. Your cars engine is controlled by this computer, also known as an ECU (electronic control unit).

What does the ECU do?

Put simply, the ECU runs a software program which is designed to allow it output signals to control the running of the engine. These outputs are varied based on how the ECU reads various inputs. So for the engine to run right the input signal must be correct. Another ability the ECU has is self diagnosis, which is where “diagnostics” comes from. When a diagnostic computer is plugged in and it communicates with the ECU, it then receives information from the ECU.

Why does my warning light come on?

The ECU is pre-programmed to check its inputs and to cross reference them against each other for the purpose of double checking. If it finds something wrong it will log a fault code and for some fault codes it will switch the warning light on. This light is commonly known as a MIL, malfunction indicator lamp.

Does a fault code tell us what the fault is?

Unfortunately the ECU is not as intelligent as many people credit it as being. It can tell, for example, if there is a fault with an electrical circuit but it cannot tell if what it is seeing is related to a component on that circuit or a wiring problem, or even if it is influenced by another circuit. This is where the mechanics knowledge and ability to test further comes into play.

Can you turn the warning light off?

Yes, it is possible to reset the warning light. This is done by clearing the fault memory in the ECU. If however, the fault has not been repaired the fault code will be set again and hence the light on the dash will return.

In summation, reading fault codes is only one minor step in the diagnosis of a fault. The use of a range of tools such as oscilloscopes, multi meters, gas analysers, wiring diagrams, data and brain power are also required.