At least once a week we get a telephone call from a prospective customer asking us for a comparison quote for a replacement part. A part they have been told needs replacing because, in many cases, a mechanic has plugged in their magic diagnosis machine and after a little witchcraft the machine has announced that the Diesel Particulate Filter is faulty, or perhaps it is the fuel rail or some other component.
Of course, we don’t offer a quote, but explain that we will only offer estimates on the basis of our own diagnoses. EOBD (Electronic On Board Diagnostics) do not offer the answers, but rather in the hands of an experienced technician they offer a useful guide to finding the fault or faults. Unfortunately, many in our industry use an EOBD print-out like a shopping list of parts required.
Let’s take an example. Recently a customer called us to ask for a price to replace the transfer box on her BMW X1. It transpired that she was at a BMW main dealer with a technician who had plugged in his EOBD and found a fault with the transfer box. Without any further investigation, without looking at the oil from the box and without considering the cause of the fault he quoted the customer £3,500.00 to replace the box.
We have had a lot of experience with BMW x-drive systems and know that transfer boxes can become clunky and noisy and throw up faults if the wrong size tyre is fitted, or if uneven tyre wear results. We advised 4 new tyres and the customer reported back to us that the problem was cured.
By far the most common example of over reliance on EOBD results relates to the rear ABS sensors on many BMW cars. Customers come to us with symptoms of ABS sensor faults (car losing power, driving ‘jumpy’, ABS and traction control lights flashing intermittently etc.) When we tell them that the fault is with the ABS sensor, it is staggering how many tell us,
“It can’t be that, I just had them replaced a week ago!”
“But I’ve had them replaced two/three/four/five times.” (Delete as your incredulity allows)
Sure, the fault will show as a faulty ABS sensor. But a simple understanding of the mechanism should lead any self-respecting technician to ask him/herself WHY the sensor is faulty.
In the case of certain BMW’s the ABS sensor is in the wheel hub, and reads the speed of the wheel from a ‘laddered’ increment wheel on the driveshaft. This wheel sits a fraction of a millimetre from the sensor. However, when it corrodes, the wheel swells and rubs against the face of the sensor, rendering it inoperable.
One can replace the sensor as often as one likes, but as soon as the car is run it will wear through the face of the sensor once more.
The solution is to remove the old increment wheel, clean up the driveshaft to prevent future corrosion, replace the increment wheel with new, and then, here at least, we paint the end of the driveshaft with rust proof paint to prevent water ingress in the future.
We have a saying among the technicians here,
“We know we’re not the cheapest, but we’re cheaper than doing it twice, or three times, or…”