Adjustment using a lambda probe (Audi A4 B5)

The catalytic converter can only operate effectively if the relative air quantity λ (lambda) is as close to 1 as possible. To achieve this, the MPI/MPFI ignition/injection system is equipped with so-called lambda control. The lambda probe measures the amount of oxygen in the exhaust gas – a comparative value for determining the composition of the fuel-air mixture. If the measured value deviates from the ideal, the control unit performs additional adjustment. The adjustment operates in the range from λ=0.8 to λ=1.2.

The lambda probe only gives a usable signal at temperatures above 350°C. In order to quickly reach this temperature, the probe is heated electrically. Until the desired temperature is reached, the system is not regulated and is guided by the specified average value λ.

The six-cylinder Audi model with all-wheel drive has two catalytic converters and two lambda probes.