Freelander 1 1.8 emissions idle test fail - Finally solved!!

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screenshot.jpg Fuel trim graph. Long term, short term and sensor 1. If anyone can explain this it would be great! I think it is the change to provide optimum fuel air mix but why 3 monitor points? 14.7 to 1 also rings a distant bell!!! This is on the 1.8k that passed emissions test.
 
This seems a good explanation: https://www.obd-codes.com/faq/fuel-trims.php

Not sure I understand your trace though (not your fault - it is what the gauge is showing) - because the long term appears to be mapping the short term fuel trim???

My hunch is that MEMS3 doesn't output the fuel trims in quite the way the reader is expecting - and there are different ways of obtaining these numbers depending on the way the tool was designed. That doesn't make sense either, because shouldn't MEMS3 be EOBD compliant?

I need to figure this out some time as I remain baffled...
 
PS part of my befuddlement is because I am thinking about MEMS1.9, which isn't EOBD. It also has LTFM and STFT.

Still don't understand why LTFT and STFT appear more-or-less the same?
 
My thoughts as to why LTFT AND STFT are close, is due to there being an error on the O2 sensor, which had upset the LTFT, until it has time to stabilise.
 
The upstream o2 sensor is the main sensor used for closed loop fuelling. The downstream
sensor is used to monitor the performance of the main catalyst and to trim the fuelling provided by the pre-catalyst
sensor.

If LTFT is adding or removing fuel, it means there is a persistent condition. You then look at what is causing the condition. Such as if LTFT is adding fuel, you look for unmetered air getting in or the amount of actual fuel being injected is not as much as the computer believes is being added based on injector on-time and fuel system pressure.
STFT on your pics detect rich exhaust and make adjustment to subtract fuel .
 
I didn't mean that STFT would be numerically the same as the Lambda - but rather the STFT will react with the Lambda (at least that would be my understanding of fuel trim: if the Lambda is rich, the STFT will tend towards lean and vice versa).
 
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