Astra VXR Lambda Sensor Diagnostics
‘How much to plug it in?’ Is a question we are often asked. It’s a common misconception that the ‘computer’ will tell us what is wrong with the car and that all we need to do is ‘plug it in’ to extract the fault code. Without going into too much detail, we have a vast amount of diagnostic equipment to aid diagnosing the issue with the vehicles. Equipment, knowledge, training and technical information are all as important as each other to carry out an actual ‘Diagnostic’.
So to the vehicle in question, we had an Astra VXR in from another garage with the engine management light on. Story we had was that the vehicle may have a catalytic convertor fault so the car was taken to a Vauxhall Dealership where two new lambda sensors were fitted. A few weeks later, the engine warning light came back on with another lambda fault at which stage the owner was told an ECU would be required.
So we read the fault code from the system and the fault code was relating to Heater Circuit Malfunction on the second (post cat) Lambda sensor, the sensor was a new, genuine Vauxhall part. I had a look at live data and noticed that the second lambda sensor was barely responsive. Even though it’s a post cat sensor, I would have still excepted some movement when snapping throttle etc. The sensor was staying at around 450MV. We gave the engine a big air leak and there was still hardly any response. My next step was to use the oscilloscope to measure the actual output at the sensor, this mirrored the live data.
Using the scope, I checked the control side of the heater circuit at the sensor and found that the ECU was controlling the sensor but only found 4 volts. I checked the supply voltage and this was battery voltage, so something wasn’t right with the voltage on the control side… Making me suspect a sensor fault.
So I connected a test light into the circuit to imitate the sensor and re-scoped, I could now see the ECU controlling the circuit and from a full 12 volts. I was now happy to condemn the new sensor.
If I had followed the ‘guided’ diagnostic system from Vauxhall, this would have told me to replace the ECU. So it’s important to understand the system your working on and have the correct inspection methods, not just working solely from the fault code.