Who is diagnosing the problem?

Source HEVRA:

There’s something of a misconception by some drivers, that the diagnosis of modern cars is done by a computer rather than a skilled individual. In fact, the computer (an essential part of the diagnostic process) just allows the technician to see things as the car sees them, and it’s down to the technician’s judgement, skill and further testing to get to the root cause, which then leads to a successful repair.

Let’s take an example of a modern vehicle, such as the Jaguar I-Pace.

If the car has a problem, a fault may have been recorded in one or more of the vehicle’s 41 addressable electronic control units (there are more than this if you consider those that don’t have their own diagnostic address).

There will generally be a number of fault codes, some that point to the cause of the problem, and some that are a consequence of the problem.

Let’s say we narrow down the fault to the Battery Energy Control Module (BECM). This ECU is capable of picking up 208 different errors.

Let’s say we have fault code P0ABF, Hybrid/EV Battery Pack Current Sensor “A” Circuit.

This fault has three possible definitions, so lets say we narrow it down to P0ABF-64, Hybrid/EV Battery Pack Current Sensor “A” Circuit- signal plausibility failure.

It’s now the technician’s job to find the root cause of the problem, so it’s out with the wiring diagram. The current sensor is located within the Battery Electrical Module, which has approximately 25 wires, all of which are red.


The current sensor itself is a control module of sorts, and although it doesn’t have its own diagnostic address, it communicates with the BECM using a controller area network. The connectors are gold plated for reliability, and pass through a ferrite core to reduce interference, before going through a connector, coming out a different colour, and finding their way back to the BECM.

The technicians needs to determine whether the fault is in the power supply, ground connection, network connections or the sensor itself. Furthermore, the fault could be a consequence of another problem- for example a partial short or a high resistance could lead to more or less current passing through the sensor than expected, in which case the fault could be somewhere else entirely. Equally, the software programming might not have allowed for the current draw of a particular circumstance, in which case the sensor may be reporting correctly and a software update may be required.

If you think this all sounds rather complicated, it’s the way of the modern vehicle, regardless of brand or even fuel type- those with combustion engines have similar levels of electronics, coupled with mechanical, thermal and fluid dynamics to consider as well.

Of course, it’s all in a day’s work for the skilled technicians in our network of garages, but hopefully this gives you some insight into why fault finding and diagnostics take the time they sometimes do and why cars can’t be diagnosed over the phone.

To ensure the best possible service to drivers, our member garages need to have a suitable qualification which ensures they know the principles and fundamentals of electric and hybrid vehicles. Our newsletters keep them up to date with new developments and allow them to gain experience from unusual problems repaired by others. Our unique vehicle-specific information means they can be familiar with a particular make and model the first time they see one, and if necessary all of our members also have access to our unlimited technical support should they need a second opinion or some help.