Imagine trying to fix a highly complex and safety critical machine, say an Airbus passenger airplane, by simply reading fault codes from a central computer and without doing any further checks or testing, simply replacing the module it seems to indicate and hoping that the problem has been fixed.
That, if you’ll pardon the pun, just won’t fly.
So why is this ‘method’ still so prevalent in the garage trade today? Apart from the obvious safety issue of letting a possibly dangerous fault go unfixed, you are quickly going to go down a path of destroying the relationship with your customer by charging them for parts they don’t need and wasting their time by forcing them to make repeats to the garage. Your reputation is also at risk if the customer then visits another garage that diagnoses the problem correctly.
Fortunately, all of this can be avoided by forming a diagnostic process and making sure you and your team use it each and every time.
Stepped process
First, talk to your customer and find out exactly what the problem that they want to have fixed. Now, we know in the real world that this is easier said than done as vehicles are sometimes dropped off out of hours or sent from other garages. We also know that what the customer states is often what they want you to hear, rather than what’s actually happening. Nonetheless, you’ll need to find out what the problem is before you try and solve it.
Next, confirm the issue. You might want to do some basic checks at this point, such as making sure the battery has the correct voltage and the terminals are tight, plus there is nothing obviously loose or broken on the vehicle.
The third step is to run a scan, and while it’s going on, check any technical service bulletins released by the manufacturer for known faults that match the symptoms.
If the above steps point to something obvious, then it is a happy day for you. Test the component to make sure it is actually faulty (and not just something that drives it, like a corroded connector or a loose earth wire), replace or repair it, and call the customer. A word of caution though: What caused that part to be faulty in the first place? Was the failure symptomatic of something else wrong with the vehicle? For example, catalyst and DPF trouble can often be symptomatic of leaky injectors and faulty or corroded electronic components can suggest that water is getting in where it shouldn’t.
Rule it out
However, if the problem wasn’t obvious from the scan, then it is time to start ruling things out. A beginner’s mistake with scan tools is to expect them to somehow highlight the code that points to the fault with flashing lights and klaxon horns… Well, they don’t work like that, and this is particularly true if the issue is a wiring fault.
Whole books have been written about the steps of the advanced diagnostic process, but a good place to start is to arm yourself with a multimeter, look up the wiring diagram on Autodata and check the circuits for power and grounds. Bear in mind that hard-to-trace intermittent electrical faults are often due to wires that are loose inside connector blocks, and poor earths might test OK when they are not under load.
There are numerous other methods of checking and testing components, but the point here is that no matter how buried the problem seems to be, using the correct vehicle data and a methodical approach will bring any fault to light eventually, no matter how obscure it seems to be. Every successful diagnosis depends on applying the same process for all problems by using a logical method and reliable data to arrive at the root cause of any issue.
Once the problem has been isolated, and you are happy that the root cause has also been dealt with, it’s time to verify the repair by testing the vehicle. After that, you can call your customer who will undoubtedly question why the charge for diagnostic time is so high compared with the actual repair. But that again is just part of the process.
