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By autotech-nic on March 2, 2020

Edward Grigg invites Steve Scott of the SimplyDiag Network to Swanley Garage Services to provide a week of ‘on the job’ training. Here, Edward details two case studies they worked through – a Land Rover Discovery with an engine management issue and a non-start Ford Fiesta.

Land Rover Discovery, Engine Management light on

Our first, and one of the most important parts of our diagnostic assessment, is questioning the customer. The owner said they had purchased their own diagnostic code reader in the hope of fixing it themselves and he presented us with a fault code for ‘bank 2 sensor 2 heated oxygen sensor circuit faulty’. He had already taken the vehicle to a well-known fast fit centre and they had changed the PCV (Positive Crankcase Ventilation) valve, which had not fixed the problem. He had also visited another local garage and they had said they didn’t know what was wrong with it either. Both garages had said to him that it was probably an air leak causing the fault and being a V8 engine, would be expensive to find.

Live data could be seen on the interactive wiring diagrams on the
mega macs 77

We read the fault codes and confirmed we had the same code as the customer. We then looked at live data and could see that the sensor wasn’t putting out a voltage reading but bank 1 was. After leaving it to run up for a few minutes, the sensor gradually started to come to life. This is the importance of knowing how these sensors work. They take a minimum of around 3000C before they start working. That is the reason they are fitted with a heater element, to assist in reaching the operating temperature faster. We carried out a resistance check on the heater circuit inside the sensor and we were not happy with the result of 3 Ohms.

As technical data was scarce on this model, we compared this to a known good result on bank 1, which was 6 Ohms. We then carried out a volt drop test on the heater circuit and confirmed that the power and ground circuit was intact. Lastly, we checked that the signal wire voltage matched what the ECU live data was showing. This test proved to be correct and proved that the sensor was faulty.

This is the importance of understanding ‘What am I testing? Why am I testing it? What am I expecting to see?’ Three basic questions that you should ask yourself before undertaking any diagnostic checks. If you are unsure on any of the above, you should find out first!

Ford Fiesta breakdown – faulty fuel rail pressure sensor?

This car was recovered into the workshop as a non-start. The customers breakdown provider had been out to it and told the customer that the fuel rail pressure sensor was faulty. The customer requested that we change this sensor, to which we explained we would need to carry out our own assessment.

We started with a full scan of the vehicle -something we always do on every car in order to see the bigger picture. There were three codes present relating to fuel rail pressure range/performance. Does this mean at this stage we should change the sensor as the breakdown assistant advised?

We looked at the live data and could see that the fuel rail pressure was close to zero. We carried out some electrical tests on the sensor and confirmed that the sensor was working correctly. We gathered some technical information and found that there was a fuse and relay located in the engine compartment fuse box that was easy to get to. We carried out our checks and found that these were OK. We then had a look to see if we could gain access to the fuel pump itself, but it was not accessible.

Using a fused jumper wire and amps clamp we were able to use a PicoScope to take a reading for the fuel pump current. We were quite alarmed by the result of 17amps. We were expecting to see around 6-7amps. We then decided to take the supply line off to the high-pressure pump and take a fuel pressure reading. This confirmed that there was no fuel rail pressure present and gave us all the evidence to ask the customer for extra time to remove the fuel tank and inspect the fuel pump.

We removed the tank and found that the fuel pump had seized. We fitted a new supply fuel pump but before giving the car back to the customer we took an amps reading from the brand new pump and saved it to our library, in case we have a fault with a similar car in the future. You can see from the screen shot of the PicoScope we have laid them on top of one another to compare good and bad.

To conclude, something I see quite frequently is people asking to change parts recommended on a fault code alone. When dealing with sensor range performance fault codes, you must assess whether it is the actual sensor at fault or is the sensor functioning correctly and the ECU is unhappy with the reading as it differs from what it is expecting. Hence, range/performance.


It’s useful to have Steve training onsite as there’s obviously no time spent away from work and I can book more diagnostic jobs in, knowing that we will get through them quickly and accurately. I can also use my own equipment and share knowledge of the various ways of using them. He also gave our reception staff advice on the importance of questioning the customer and how best to sell diagnostic time.

We were able to do a group training session that included all my technicians and they gave great feedback on how Steve had inspired them. All in all, I highly recommend his private training.



About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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