Lambda Output Voltage
Figure 7- The interactions of various quantities in the Lambda control system
In practice, whilst the lambda sensor is being used in closed loop mode, the output voltage of the sensor will swing back and forth, a few times every second.
This is because of the ECU using the lambda sensor's output to cycle the mixture back and forth, from rich to lean and back again, continuously. This is to provide the optimum conditions for the catalyst to operate correctly.
The output voltage of the sensor has a very large step around the stoichiometric point, like a switch.
Figure 8- Lambda Sensor voltage output curve
A malfunctioning, degraded, or worn out sensor will have this central point in the wrong place, or it's response time will be sluggish rather than fast. A new sensor will switch in about 0.2 seconds. A worn out sensor may take as long as 1.5 seconds to switch.
This will confuse or misdirect the ECU and result in poor running.
Emissions will be up, and an MOT failure is a probability. Ultimately, catalyst damage could occur if the system spends too much time over-fuelling.