Hi, newbie to the forum and I am trying to investigate why my Elise is running rich, it has MEMS 1.9. I get Oxygen Sensor Status Okay for approximately 5 minutes then it switches to Not Okay. Does anyone know what parameters change to trigger the okay to not okay status on the diagnostic tool?
The "okay" to "not okay" does not come from the diagnostic tool but from the ECU itself.
pscan is simply telling you what the ECU is saying.
So the answer to your question is that the Oxygen Sensor Status parameter inside the MEMS1.9 ECU changed from "ok" to "not okay".
The only people who can tell you exactly what triggers this inside the ECU are probably the MG Rover engineers who designed the MEMS1.9.
On a normally running engine that's in closed loop mode (which it should be when idling) the lambda sensor should continuously fluctuate between I think about 0.2V and 0.9V. The sensor can only tell the ECU that the mixture is either leaner or richer than it should be (it's a digital sensor so either on or off). The ECU therefore will continuously adjust the fueling "up a bit", "down a bit", "up a bit", "down a bit" and that's why the sensor voltage should fluctuate like that.
If it doesn't then something is wrong.