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.
DIAGNOSTIC CASE STUDY 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.
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
We carried out a resistance check on the heater circuit inside the sensor and we were not happy with the result of 3 Ohms.
Live data could be seen on the interactive wiring diagrams on the mega macs 77
DIAGNOSTICS
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.
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