The program section self-diagnosis within the control unit, checks the sensor signals against pre-programmed, 'look-up' tables. If an open circuit should occur say, in the temperature sensor, this would result in an infinite resistance. The corresponding temperature of -35 °C is not plausible. The malfunction is therefore recognised.
Similar plausibility controls are carried out with other sensor signals.
A fault which has been recognised is stored with the corresponding trouble code.BACK-UP OPERATION
If a fault does occur, the control unit substitutes the following values so that the vehicle is not immobilised.
Oxygen sensor circuit - 450 mV
Coolant temperature sensor - 40 °C
Knock sensor circuit Ignition timing is retarded by 8.5°
Throttle valve sensor malfunction - Throttle angle 30°
Intake air temperature sensor - 20 °C
Hot wire mass air flow meter - Replacement characteristic curve, dependent on engine speed and throttle valve angle.