Determining Oxygen Sensor Status Okay / Not OK

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?

Hi,

I only just got Pscan and I am looking at data from MEMS3 on a MG TF.

I don't see an "Oxygen Sensor Status Okay" value but I do see a "Oxygen Sensor Condition" value, might be the same.

I am trying to figure out why my engine is running rich with high CO (just failed an MOT).
I get,
"Oxygen Sensor Condition" = Normal
"Oxygen sensor switch state" = Switching from lean to rich
"Upstream Oxygen Voltage" = 0.10 to 0.75 (starting to think this might be the problem, if it's very rich, I'm sure it should be 0.9v or more.

Is the voltage of the O2 sensor on Elise changing much from 0.45V, if not it might be dead, may be the reason for the "Not OK" value. Do you get a sensor switch state value, should be switching from rich to lean, as the ECU adjusts the Fuel trims.

DISCLAIMER - I only really know enough about all this to get my self into trouble.

Iain.

Thanks Iain, the O2 sensor is running at 0.95V or thereabouts as the engine is cycled. I’m trying to find out what things change, or thresholds are breached, to get that Okay to not Okay displayed on the diagnostics. MEMS3 lookis to have some additional/different parameters. Good luck with your quest too to get emissions more reasonable.

Hi and welcome to the forum

How are you reading the output?

James

Hi James,

I’m reading the output from the pscan diagnostic scanner set on Petrol MEMS 1.9, field Oxygen Sensor Status. All the best, Paul.

Paulanton wrote:

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.

Many thanks for the info, much appreciated, so the status change is within logic within the ecu itself. Thanks.

I always thought that system ran in open loop at idle, until the oxygen sensor hit 350 degrees Centigrade, and or is being driven.
At idle from cold, I get a set Air fuel ratio of 14.7:1 , open loop and voltage of 0.44 v, other readings regarding the oxygen sensor are not available as it is in open loop .
I am assuming if I drive the car until the oxygen sensor warms up, I will achieve closed loop and then I will see the sensor switching !

If the sensor is not up to temperature, the mixture will not be accurately read and if the engine temperature sensor is not reading accurately, (a fairly common occurence,) it will probably be reading the engine as cold and therefore overfuelling. Most amateur diagnosticians see a lambda reading that is not as expected and assume that the sensor is telling lies. The reality is that the sensor is reacting to a genuine fault. If lean, check for air leaks first, exhaust leaks second. If rich, check the operation of temperature sensors.