Self Diagnostic Fault Codes on K11

Has anyone tried using the ECM diagnostic procedure for retrieving the fault code(s) from the ECU on a post 1999 K11? I've got some info off the net about how to do this, but wondered if anyone has already had any success. My engine management light has been on for nearly 4 years and during that time, 2 reputable garages have ran the full computer check and found nothing. The car is running well but I am pretty certain that it's performance has gone down hill slightly since 2 years ago. It was hardly used for the best part of 1 year. The throttle body is my main area of concern. Car never stalls or judders and fuel economy is good, but there is something just not quite right.
Also, I read about a guy who literally just removed the lambda sensor and replaced it with a new equivalent....... didn't think it was that straight forward, thought it had to be re-setup. Any advice? By the way, many thanks to the 3 people who responded to my recent posts regarding emulsified oil, PCV filter and alternator brushes. (Y)

Thanks Frank. This is the site where I originally found the info. Just a bit dubious about messing with the vehicle computer. Presumably the site is providing reliable info. Thanks once again.