Getting The Correct Diagnosis From Your Mechanic

The Wrong Diagnoses Can Cost You Dearly

It’s important that the garage and mechanics you use know thier stuff


Case Study


We gained a new customer earlier this year by diagnosing an issue another garage had failed to do correctly, the customer had their car brought to us on the back of a low loader as it wasn’t starting, and when it was, the electrics were going haywire. It came to us after the other garage had stated what they thought the fault was, this particular garage works exclusively on this specific brand of vehicle, only employ Master Techs trained by the vehicle manufacturer and would be classed as subject matter experts.


The customer was told by the garage that they needed 2 new ECUs which would be £1200+ to supply and fit them, the customer had decided to get a second opinion before forking out that amount of money.


The car was inspected by us and we determined there was an issue with the electrical system. We used wiring diagrams, scantools and multimeters to check for values and compared them to known good readings, tested components and narrowed down each area to where the fault lay. This led us to focus the search area to a specific wiring harness and components in the harness, we traced the harness from under the fuse box, removed the protective shielding and found this (see image).



A severely corroded wiring harness that fed power to the vehicle's multiple computers

View of the protective shield removed from around corroded wiring harness

The images above show a heavily corroded wiring harness, this wiring harness takes a direct feed from the car battery and goes to multiple ECUs, it sits under the fuse box and against the body of the vehicle. The corrosion was caused by water ingress aswell as not being sufficiently sealed at the factory.


The harness was broken at the splice of wires causing conectivity issues, the symptoms; the car lights were erratic, failing to start, electronic handbrake not releasing, multiple codes present for ECU communication errors, speakers not working, reverse sensors going crazy and a dashboard that would cut out intermittently, all controlled by the car computers.


The harness was repaired by removing the corroded section of broken wires, soldered and heat shrink applied to the splice, the water leak coming from the boot was also corrected. This was done at a cost of less than 80p in materials and a few hours labour.


If the customer had went with the original diagnosis, the fault would not have been rectified and only an expensive part changed for absolutely no reason, mechanics rely heavily on scan tools for information but they can lead us down the wrong direction. Knowing how the systems operate and a clear, methodical approach is what solves issues.


This customer was fortunate they did not spend the money, otherwise it would have been an expensive outlay and nothing would have changed. We wish we could say this was the first occasion this has happened but it is a regular occurence.


Trading Standards are very helpful if information is required regarding a garage charging for a wrong diagnosis/repair and what your rights, as a consumer, are.