[Gav]

The ignition sensor is a hall effect sensor. Most late model cars speedo use the same principal too.
The closed loop that Nasty is refering to in broad terms, is the ECU's ability to fine tune itself according to what readings it is getting from the sensor in the exhuast and other sensors on the motor.
Not running a closed loop would see your motor running to fixed fuel and ignition settings according to throttle position, vacuum and temperature ect and not being able to make subtle changes to keep things like fuel mixture running at their optimum.
Sorry to hear about your test on friday Mando, when will you be getting it retested?