I can't find a code for a specific parameter

Be aware that there are much more parameters in P01 than in P09. So be sure to use the parameter list which answer your needs.

Nevertheless, it may happen than the parameter you need does not exists in both P09 or P01 lists.

In that case write a message to sdn-userdesk, describing in details the parameter that you have (name, unit and method for analysis if available).