The use of wiring diagrams in the process of diagnosing faults is a very valuable tool often over looked by many workshops. Here we explain how misdiagnosis of a fault can lead to costly and often unnecessary repairs.

When a car is first brought into any modern workshop, one of the first tasks is to connect the vehicle to a computer via its’ diagnostic socket under the dashboard and run a full scan on all systems.

Depending on the car and it’s age and condition – this will generally show a number of both current and pending fault codes.

One such test on a car in our workshop recently showed three faults in the Engine Control Unit (ECU). Our technician quickly established that the three faulty items were unlikely to have failed all at once. The diagnostic tool showed:

  1. A faulty Air Mass Airflow Sensor / Meter (MAF)
  2. A faulty Exhaust Recirculating Valve (ERG)
  3. A faulty Purge Control Valve.

It would seem logical then to just replace these parts. However, the combined costs of these parts is considerable – so we take the time to conduct further analysis to get as precise a picture of what’s actually going on as possible.

Advanced Diagnostics / Wiring Diagrams Coby Autos -
Advanced Diagnostics - Using Wiring Diagrams to pinpoint the root cause of engine problems

We do this by pulling up a wiring diagram on our online access systems to look for any correlation between the three items listed as faulty. In this particular case, we saw that the three parts showing as faulty shared a common power supply.

Upon closer inspection – and following the path of the power source as the next step – we found that the power came via a fuse located under the bonnet in the main fuse box. A simple fuse being blown can cause major problems – and as it turned out the fuse was gone, so we replaced it.

However, we found that it immediately blew again. We then quickly disconnected all three items in our fault list, and replaced the fuse a third time. The fuse did not blow again, so we reconnected each item in turn to see which sensor causing the issue.

It turned out to be the ERG valve, so only one part was required to fix this car.

Maurice says... Maurice Coby of Coby Autos

Our workshop frequently receives cars for repair from customers who have gone elsewhere and had a myriad of new parts fitted – at great expense – and find their problems not fully solved. Understandably they’re often very frustrated.

We also regularly have cars sent onto us from other garages who haven’t quite the range of diagnostic tools we at Coby Autos have – or the experience and expertise that 30 years in this business gives you.

We use state of the art equipment to ensure that we fully diagnose a problem before throwing parts and labour at it – ensuring that you spend your money in the right place and only on the parts that need changing and work that is actually required.